github: Switch to downloading GNU sources from a specific mirror

Fetching from ftpmirror.gnu.org, from the github actions runners,
returns "502: Bad Gateway" some times during the day, while it seems
to work fine other times.

Fetching directly from ftp.gnu.org currently seems to work fine,
although we switched away from that earlier in
eea8f577e02a08d90e7a7c1c60251e7760f6a880, when ftp.gnu.org was
overloaded at the time.

Fetch from a trustworthy mirror, to avoid relying too much on the
also possibly overloaded main server ftp.gnu.org.

Signed-off-by: Martin Storsjö <martin@martin.st>