[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>

.github/workflows/build.yml

@@ -49,7 +49,8 @@ jobs:
           sudo apt update
           sudo apt install -y -o Acquire::Retries=50 \
                            mtools syslinux isolinux \
-                           libc6-dev-i386 libc6-dbg:i386 valgrind
+                           libc6-dev-i386 valgrind \
+                           libgcc-s1:i386 libc6-dbg:i386
       - name: Build (BIOS)
         run: |
           make -j 4 -C src