Fix various UTF-8 encoding bugs

[fmeum]

Fixes missing reencodings from Bazel's internal representation to UTF-8 or vice versa:

• environment variables for locally executed spawns when the JVM uses a UTF-8 locale
• output file paths for remote cache uploads
• symlink targets

Also renames the encoding/decoding methods to make it more clear which one is correct for which situation and inline them into RemotePathResolver to get rid of a common gotcha.

[fmeum]

@tjgq I found these bugs while working on the output inlining feature for remote execution.

[fmeum]

Tests are passing again, I forgot to use the reencode method in one spot.