Require bash for rgbenv

[ZoomTen]

Enforces bash to run rgbenv. Under sh, it first checks for the existence of bash on the device, and then re-executes itself under bash if it exists.

On slower environments, e.g. Termux, it might cause a slight delay on startup. However it should have trivial effect on the "usual" (PC, etc.) environments.

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Although bash is the most commonly-used shell (especially on Linux), I was concerned about other environments (outside of WIndows with batch files) where that may not be available (see #9).

But I think I want the extra checking facilities that bash provides, and honestly POSIX sh is a bit too limiting. I'm concerned about Mac OS since I'm still unable to test that, but I'd like to hear some thoughts on this.

[ZoomTen]

I suspect the "re-exec" method may have tripped the CI and froze the tests. Seems to work well on my end :(

Will figure out something.

[ISSOtm]

macOS provides bash always, so that should be fine?

[ZoomTen]

It's an old version though, is it? I probably might have used features of the newer version without even realizing it...

[ISSOtm]

Hmm, good point. Does ShellCheck have a feature for that?