ci: set GIT_COMMITTER_DATE in a locale-agnostic format

When adding patches wit git-am create the date format for
GIT_COMMITTER_DATE in a locale-agnostic format, this avoids potential
issues when running the scripts locally with a non-English locale, e.g.:

-----------------------------------------------------------------------
++ GIT_COMMITTER_DATE='gio 1 gen 1970, 01:00:00, CET'
++ git am
fatal: invalid date format: gio 1 gen 1970, 01:00:00, CET
-----------------------------------------------------------------------

Reviewed-by: Eric Engestrom <None>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30895>

.gitlab-ci/container/build-angle.sh

@@ -30,7 +30,7 @@ angle_patch_files=(
 )
 for patch in "${angle_patch_files[@]}"; do
   echo "Apply patch to ANGLE from ${patch}"
-  GIT_COMMITTER_DATE=$(date -d@0) git am < "${ANGLE_PATCH_DIR}/${patch}"
+  GIT_COMMITTER_DATE="$(LC_TIME=C date -d@0)" git am < "${ANGLE_PATCH_DIR}/${patch}"
 done
 
 {