Force locale setting for the script

[ehrenfeu]

This avoids localized output that can confuse the script, e.g. when determining
the $defaultBranch.

We (as in @lguerard and me) were facing issues when trying to create a new
release using a fr_FR locale setting on Linux (Mint 20.1 / Ubuntu 20.04) as the
command for getting the default branch is failing there (the grep pattern doesn't
match on the french locale):

scijava-scripts/release-version.sh

Line 186 in 8bd74a0

defaultBranch=$(git remote show "$remote" | grep "HEAD branch" | sed 's/.*: //')

PLEASE NOTE: this is entirely untested on MacOS and Windows, although
in my experience using the C locale should be supported there as well.

[ctrueden]

Thanks, @ehrenfeu! This change looks safe to me, so I'll go ahead and merge it.