subprocess: Forward proxy env for postinstall script

[pothos]

The proxy env vars for curl can be set up for the update-engine service but so far weren't forwarded to the postinstall script. Now that we download from it, we need to forward them.
Forward the env vars for curl and ue-rs download_sysext (Flatcar Stable currently uses curl, Alpha uses ue-rs).

How to use

Backport to Stable and LTS.

Testing done

Checked that these env vars pop up when set up. First I tested

# /etc/systemd/system/update-engine.service.d/override.conf
[Service]
Environment="LD_LIBRARY_PATH=/usr/lib64"

with sudo flatcar-update … and it made one more env var entry (2 vars). The same should happen for the proxy vars.

sudo strace -f -e trace=execve -p $(pgrep update_engine) 2>&1 | grep postinst
[pid 163396] execve("/tmp/au_postint_mount.DzIv8e/postinst", ["/tmp/au_postint_mount.DzIv8e/pos"..., "/dev/vda4", "KERNEL=/boot/flatcar/vmlinuz-b"], 0x5601b610aaf0 /* 2 vars */) = 0

With setting Environment=HTTPS_PROXY=http://a.b on a patched system and running sudo ./flatcar-update -V 3850.0.0 -F with this patch to have it download the payload as fallback:

$ diff -u /usr/bin/flatcar-update flatcar-update 
--- /usr/bin/flatcar-update	2024-01-24 11:38:22.000000000 +0000
+++ flatcar-update	2024-01-24 13:40:51.383160024 +0000
@@ -113,7 +113,7 @@
 fi
 
 # Use the old mount point for compatibility with old instances, where the script gets copied to
-OEMID=$({ grep -m 1 -o "^ID=.*" /usr/share/oem/oem-release 2> /dev/null || true ; } | cut -d = -f 2)
+OEMID= # $({ grep -m 1 -o "^ID=.*" /usr/share/oem/oem-release 2> /dev/null || true ; } | cut -d = -f 2)
 
 # Determine what to download from release server if no local payload is given.
 # Using /usr/share/flatcar/oems/ from the currently running version means the download is only best-effort

we see the 2 vars strace log as well, indicating that the env var is forwarded, and we get a failed update because the fallback download can't reach https with a non-existing proxy:

Jan 24 13:42:57 localhost update_engine[2565]: I0124 13:42:57.641688  2565 subprocess.cc:43] Subprocess status: 1
Jan 24 13:42:57 localhost update_engine[2565]: I0124 13:42:57.641752  2565 subprocess.cc:46] Subprocess output:
Jan 24 13:42:57 localhost update_engine[2565]: Args { output_dir: "/var/lib/update_engine/ue-rs/", input_xml: Some("/var/lib/update_engine/prefs/full-response"), payload_url: None, pubkey_file: "/usr/share/update_engine/update-payload-key.pub.pem", image_match: ["oem-qemu.gz"], print_progress: false }
Jan 24 13:42:57 localhost update_engine[2565]: mv: cannot stat '/var/lib/update_engine/ue-rs//oem-qemu.raw': No such file or directory
Jan 24 13:42:57 localhost update_engine[2565]: Falling back to release-server for extension 'oem-qemu'
Jan 24 13:42:57 localhost update_engine[2565]: Args { output_dir: "/var/lib/update_engine/ue-rs/", input_xml: None, payload_url: Some("https://update.release.flatcar-linux.net/amd64-usr/3850.0.0/oem-qemu.gz"), pubkey_file: "/usr/share/update_engine/update-payload-key.pub.pem", image_match: [], print_progress: false }
Jan 24 13:42:57 localhost update_engine[2565]: Error: unable to download data(url Url { scheme: "https", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("update.release.flatcar-linux.net")), port: None, path: "/amd64-usr/3850.0.0/oem-qemu.gz", query: None, fragment: None })
Jan 24 13:42:57 localhost update_engine[2565]: Caused by:
Jan 24 13:42:57 localhost update_engine[2565]:     0: client get & send"https://update.release.flatcar-linux.net/amd64-usr/3850.0.0/oem-qemu.gz" failed
Jan 24 13:42:57 localhost update_engine[2565]:     1: error sending request for url (https://update.release.flatcar-linux.net/amd64-usr/3850.0.0/oem-qemu.gz): error trying to connect: dns error: failed to lookup address information: Name or service not known
Jan 24 13:42:57 localhost update_engine[2565]:     2: error trying to connect: dns error: failed to lookup address information: Name or service not known
Jan 24 13:42:57 localhost update_engine[2565]:     3: dns error: failed to lookup address information: Name or service not known
Jan 24 13:42:57 localhost update_engine[2565]:     4: failed to lookup address information: Name or service not known
Jan 24 13:42:57 localhost update_engine[2565]: Failed to download required OEM update payload

The same happens with sudo ./flatcar-update -V 3760.2.0 -F which uses curl instead of ue-rs (the error message is curl: (5) Could not resolve proxy: a.b).