revert to gcc-13 for linux-glibc-debian-testing-gcc

To avoid this:
```
+ apt-get --option Dpkg::Use-Pty=0 install --yes curl git gpg gpg-agent rsync python3-pefile make cmake zip xz-utils time jq secure-delete checksec qemu-user-static gcc-14 g++-14 gcc-14-aarch64-linux-gnu g++-14-aarch64-linux-gnu libgcc-14-dev libc6-dev-arm64-cross
Reading package lists...
Building dependency tree...
Reading state information...
Package gcc-14-aarch64-linux-gnu is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gcc-14-aarch64-linux-gnu' has no installation candidate
E: Unable to locate package g++-14-aarch64-linux-gnu
E: Couldn't find any package by regex 'g++-14-aarch64-linux-gnu'
```
Ref: https://github.com/curl/curl-for-win/actions/runs/8985369476/job/24679216957#step:3:104

Interestingly the same works when musl is also installed.

.github/workflows/build.yml

@@ -98,6 +98,7 @@ jobs:
  export CW_CONFIG='${{ github.ref_name }}-linux-gcc'
  export CW_REVISION='${{ github.sha }}'
  . ./_versions.sh
+ export CW_GCCSUFFIX='-13'
  docker trust inspect --pretty "${DOCKER_IMAGE}"
  time docker pull "${DOCKER_IMAGE}"
  docker images --digests