Exec: use C instead of en_US.UTF-8 to set the Exec locale.

[tmds]

The en_US locale can't be used on systems where it is not installed. This is common in container images.

On such systems, setting the locale to en_US.UTF-8 causes unexpected warnings to be written to standard error.

When Exec.LogStandardErrorAsError is set, these warnings cause the Task to fail due to logging errors.

This changes to use the 'Computer English' C.UTF-8 locale, which is always available.

Fixes #4194

@rainersigwald @wfurt @janvorli ptal

cc @mthalman @omajid

[wfurt]

LGTM. thanks.

[tmds]

This was not meant to be merged yet, as we were still discussing the options in #4194. I should have made it more apparent in this PR.

Based on the discussion there, leaving out the LANG/LC_ALL (what was merged) is a good option.

• We're no longer setting something that may be unsupported (fixes Exec tasks warns: bash : warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) #4194).
• .NET assumes Unix platforms to have UTF-8 as the default encoding, so we can do the same rather than try to enforce it.
• We're now using the system locale on Unix for the tool process. This is the same as how it behaves on Windows.

@rainersigwald @wfurt @janvorli @danmoseley @KalleOlaviNiemitalo if it is good for you, I think we can keep this and see what feedback we get during the .NET 9 development.

[rainersigwald]

Our main is currently flowing into 8.0.200 builds as well as 9.0.100 builds so I think we should be cautious and revert for now. Sorry for the confusion!