matrix: add arm64 install-only builder

This is a step-1 to supporting arm64 a bit better in CI.  If this is
fast enough, maybe we can get some occasional smoke tests on top of it
or something but it should make getting dev containers going easier at
the least.

src/test/generate-matrix.py

@@ -149,6 +149,16 @@ def __str__(self):
     docker_tag=True,
 )
 
+# Debian: arm64, expensive, only on master and tags, only install
+# if matrix.branch == 'master' or matrix.tag:
+matrix.add_build(
+    name="bookworm - arm64",
+    image="bookworm",
+    platform="linux/arm64",
+    docker_tag=True,
+    command_args="--install-only ",
+)
+
 # Debian: gcc-12, content-s3, distcheck
 matrix.add_build(
     name="bookworm - gcc-12,content-s3,distcheck",