(20.lts.1+) Run ODT jobs on dedicated runners (#2088)

Make on-device test jobs run on dedicated runners that use very little
CPU and RAM. This should free up linux runners and reduce job queueing.

b/298255792

.github/workflows/main.yaml

@@ -93,7 +93,7 @@ jobs:
   # Builds, tags, and pushes Cobalt docker build images to ghr.
   docker-build-image:
     needs: [initialize]
-    runs-on: [self-hosted, linux, X64]
+    runs-on: [self-hosted, linux-runner]
     permissions:
       packages: write
     steps:
@@ -127,7 +127,7 @@ jobs:
     needs: [initialize]
     permissions:
       packages: write
-    runs-on: [self-hosted, linux, X64]
+    runs-on: [self-hosted, linux-runner]
     steps:
       - name: Checkout files
         uses: actions/checkout@v3
@@ -156,7 +156,7 @@ jobs:
   # Runs builds.
   build:
     needs: [initialize, docker-build-image]
-    runs-on: [self-hosted, linux, X64]
+    runs-on: [self-hosted, linux-runner]
     permissions: {}
     name: ${{matrix.name}}_${{matrix.config}}
     strategy:
@@ -190,7 +190,7 @@ jobs:
   on-host-test:
     needs: [initialize, docker-unittest-image, build]
     if: needs.initialize.outputs.on_host_test == 'true'
-    runs-on: [self-hosted, linux, X64]
+    runs-on: [self-hosted, linux-runner]
     permissions: {}
     name: ${{matrix.name}}_${{matrix.shard}}_test
     strategy: