[ci] Work around Ubuntu packaging metadata issues

[ci] Work around Ubuntu packaging metadata issues

The libc6-dbg:i386 package has spontaneously started failing to
install from the Azure package repositories used by the GitHub Actions
runners, with the somewhat recalcitrant error message:

 libc6:i386: Depends: libgcc-s1:i386 but it is not going to be installed

Work around this unexplained issue by explicitly requesting
installation of the libgcc-s1:i386 package.

Signed-off-by: Michael Brown <mcb30@ipxe.org>