GitHub Actions: increase git POST size to clone systemd

Building `systemd-selinux` fails with:

    ==> Retrieving sources...
      -> Cloning systemd-stable git repo...
    Cloning into bare repository '/startdir/systemd-selinux/systemd-stable'...
      -> Cloning systemd git repo...
    Cloning into bare repository '/startdir/systemd-selinux/systemd'...
    error: RPC failed; HTTP 400 curl 22 The requested URL returned error: 400
    ==> ERROR: Failure while downloading systemd git repo
        Aborting...

More precisely, `git clone --mirror` fails:

    $ git clone --origin=origin --mirror https://github.com/systemd/systemd --verbose
    Cloning into bare repository 'systemd.git'...
    POST git-upload-pack (169 bytes)
    POST git-upload-pack (chunked)
    error: RPC failed; HTTP 400 curl 22 The requested URL returned error: 400

Increase the size of POST buffer to work around this:

    $ git clone --origin=origin --mirror https://github.com/systemd/systemd -c http.postBuffer=536870912 --verbose
    Cloning into bare repository 'systemd.git'...
    POST git-upload-pack (169 bytes)
    POST git-upload-pack (gzip 2502852 to 1211683 bytes)
    remote: Enumerating objects: 956381, done.
    remote: Counting objects: 100% (10445/10445), done.
    remote: Compressing objects: 100% (2150/2150), done.
    ...

.github/workflows/main.yml

@@ -33,6 +33,7 @@ jobs:
           chown -R builduser /startdir /packages /build /logdest && \
           sudo -u builduser /startdir/clean.sh && \
           sudo -u builduser /startdir/recv_gpg_keys.sh && \
+          sudo -u builduser git config --global http.postBuffer 536870912 && \
           sudo -u builduser /startdir/build_and_install_all.sh && \
           rm -rf /startdir/*/src/ /startdir/*/pkg/ && \
           pacman --noconfirm -Sc && rm -rf /var/cache/pacman/pkg/* && \