Add a CI job to run specs with LC_ALL=C

[eregon]

• Which simulates having no locale set and using the "default" locale.
• To catch failures earlier rather than in https://rubyci.org/
• See Add specs for all of IO::Buffer constructors #1305 (comment)

[eregon]

Out of curiosity I checked which locales are available and which are default on GitHub Actions in https://github.com/eregon/actions-shell/actions/runs/21473041904/job/61850086645:
ubuntu-22.04 and ubuntu-22.04-arm:

$ locale
LANG=C.UTF-8
LANGUAGE=
LC_CTYPE="C.UTF-8"
LC_NUMERIC="C.UTF-8"
LC_TIME="C.UTF-8"
LC_COLLATE="C.UTF-8"
LC_MONETARY="C.UTF-8"
LC_MESSAGES="C.UTF-8"
LC_PAPER="C.UTF-8"
LC_NAME="C.UTF-8"
LC_ADDRESS="C.UTF-8"
LC_TELEPHONE="C.UTF-8"
LC_MEASUREMENT="C.UTF-8"
LC_IDENTIFICATION="C.UTF-8"
LC_ALL=

$ env | grep -E 'LC|LANG'
LANG=C.UTF-8

$ locale -a
C
C.utf8
POSIX
en_US.utf8

macOS 15 arm & intel:

$ locale
LANG="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_CTYPE="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_ALL="en_US.UTF-8"

$ env | grep -E 'LC|LANG'
LC_ALL=en_US.UTF-8
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8

$ locale -a
...
C.UTF-8
en_US
en_US.UTF-8
en_US.ISO8859-1
en_US.ISO8859-15
en_US.US-ASCII
...
C
POSIX