Start on trying to get an s390x git in the s390x cross container

Currently this fails with:

    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies:
     git:s390x : Depends: perl:s390x but it is not going to be installed
                 Depends: liberror-perl:s390x but it is not installable

Or, in some variations, the (same) error is expressed as:

    E: Package 'liberror-perl:s390x' has no installation candidate

Author: EliahKagan

etc/docker/Dockerfile.test-cross

@@ -2,17 +2,26 @@ ARG TARGET
 
 FROM ghcr.io/cross-rs/${TARGET}:latest
 
-RUN apt-get update && \
+ARG TARGET
+
+RUN set -uC && \
+    apt_suffix= && \
+    if target_arch="$(dpkg-architecture --host-type "$TARGET" --query DEB_HOST_ARCH)"; then \
+        dpkg --add-architecture "$target_arch" && \
+        apt_suffix=":$target_arch"; \
+    fi && \
+    apt-get update && \
     apt-get install --no-install-recommends -y apt-transport-https gnupg && \
     release="$(sed -n 's/^VERSION_CODENAME=//p' /etc/os-release)" && \
     echo "deb https://ppa.launchpadcontent.net/git-core/ppa/ubuntu $release main" \
         >/etc/apt/sources.list.d/git-core-ubuntu-ppa.list && \
     apt-key adv --keyserver keyserver.ubuntu.com \
         --recv-keys F911AB184317630C59970973E363C90F8F1B6217 && \
     apt-get update && \
-    apt-get install --no-install-recommends -y git jq patch && \
+    apt-get install --no-install-recommends -y "git$apt_suffix" jq patch && \
     rm -rf /var/lib/apt/lists/* && \
-    to_patch=/android-runner && \
-    patcher='s/^export LD_PRELOAD=/test "${NO_PRELOAD_CXX:-0}" != 0 || &/' && \
-    if test -f "$to_patch"; then sed -i.orig "$patcher" -- "$to_patch"; fi && \
+    if test -f /android-runner; then \
+        sed -i.orig 's/^export LD_PRELOAD=/test "${NO_PRELOAD_CXX:-0}" != 0 || &/' \
+            /android-runner; \
+    fi && \
     git config --system gitoxide.imaginary.arbitraryVariable arbitraryValue