ci: Test x86_64-musl with musl 1.2.3

We already test aarch64 and a few other platforms, but x86 was missing.

(backport <#4761>)
(cherry picked from commit 8cac94f)

Author: tgross35

.github/workflows/ci.yaml

@@ -225,6 +225,9 @@ jobs:
           - target: powerpc64le-unknown-linux-musl
             env:
               RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1
+          - target: x86_64-unknown-linux-musl
+            env:
+              RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1
           # FIXME(ppc): SIGILL running tests, see
           # https://github.com/rust-lang/libc/pull/4254#issuecomment-2636288713
           # - target: powerpc-unknown-linux-gnu