Tests: fix RunCMake.Make test when run on systems with non-english locale

Set LANG=C in the environment so the output of make is predictable.

Otherwise the test fails like this (truncated):

346:   Expected stdout to match:
346:
346:    expect-out> .*Nothing to be done for.*hello.*
346:
346:   Actual stdout:
346:
346:    actual-out> gmake[2]: Für das Ziel „CMakeFiles/hello.dir/build“ ist nichts zu tun.

Fixes: #19689 (closed)
Backport: release