Enforce gcc~builtins (eth-cscs#284)

With spack/spack-packages#3106 a default value
change for `gcc` variant `binutils` has been introduced.

This triggers a problem (see [CI
error](https://gitlab.com/cscs-ci/ci-testing/webhook-ci/mirrors/551234120955960/1440398897047560/-/jobs/13261281018#L447))
when building `binutils`, which by default is built `libs=static,shared`
but we explicitly prefer `zstd libs=static`.


https://github.com/eth-cscs/stackinator/blob/c7db13cd3c12d595cf6a7793db88603db9495889/stackinator/templates/compilers.gcc.spack.yaml#L22-L23

The proposed change does not fix the root problem, but it simply prefers
building `gcc~builtins` as we've always did, completely workarounding
the underlying problem.

I'm going to address the root issue in spack, but for what concerns
stackinator, at the moment IMHO this is the most reasonable and simple
solution.

Author: albestro

stackinator/templates/compilers.gcc.spack.yaml

@@ -12,7 +12,7 @@ spack:
     reuse: false
   packages:
     gcc:
-      variants: [build_type=Release +bootstrap +profiled +strip]
+      variants: [build_type=Release +bootstrap +profiled +strip ~binutils]
     mpc:
       variants: [libs=static]
     gmp: