Fix cross-compilation from macOS

We can't use `LC_CTYPE=C sed` there since /usr/bin/sed is not compatible
with the expressions in nls/ (`sed: RE error: illegal byte sequence`).
To fix this use $(SED) instead which autotools will set to a GNU
version of sed (usually /usr/local/bin/gsed) on macOS.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
4 jobs for fix-macos-crosscompile in 19 minutes and 9 seconds
Status Name Job ID Coverage
  Prep
passed check-commits #10872684

00:00:49

passed container-prep #10872685

00:12:25

 
  Build
passed build #10872686

00:06:13

 
  Deploy
failed check-merge-request #10872687
allowed to fail

00:00:30

 
Name Stage Failure
failed
check-merge-request Deploy
  File "/usr/local/lib/python3.9/site-packages/gitlab/exceptions.py", line 306, in wrapped_f
raise error(e.error_message, e.response_code, e.response_body) from e
gitlab.exceptions.GitlabGetError: 404: 404 Project Not Found
Uploading artifacts for failed job
Uploading artifacts...
WARNING: results.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1