rsvg-test.c: Override the environment's locale with LC_ALL

We use a made-up set of language preferences for tests with the
systemLanguage attribute.  Previously we were using the LANGUAGE
environment variable, which is a GNU extension.

Now we use LC_ALL, which is the "override everything" standard variable.
3 jobs from system-language in 8 minutes and 15 seconds (queued for 35 minutes and 23 seconds)
Status Job ID Name Coverage
  Test
passed #117343
fedora test

00:07:52

 
  Lint
manual #117345
allowed to fail manual
clippy
passed #117344
rustfmt

00:00:23