Force a UTF-8 locale when `en_US.ISO-8859-1` is not available

[fmeum]

This provides UTF-8 support on systems that don't have valid locale settings by default and also don't provide the en_US.ISO-8859-1 locale.

Also show a warning if Bazel fails to force such a locale and falls back to ASCII.

Fixes #26259

[fmeum]

@hanwen-flow Could you test this PR on your machine? I would like to make sure that it actually resolves the issue for you and doesn't just show a warning.

[hanwen-flow]

@fmeum - I'll try to have a look early next week.

[fmeum]

I manually verified that this results in sun.jnu.encoding = UTF-8 on a Linux machine without the en_US.ISO-8859-1 locale installed and with LC_ALL=C set on server startup, so I'm positive that this does fix the issue.

[cgrindel]

Can we merge this issue soon? It is keeping me from enabling last_green tests on a couple of my repositories. Thanks!

[fmeum]

@tjgq Friendly ping

[tjgq]

@tjgq Friendly ping

Sorry for the delay - it's under internal review now

[fmeum]

@bazel-io fork 8.4.0