ci: add more ARM build configurations to CI and release workflows

... it turns out that rustembedded/cross:armv7-unknown-linux-musleabi
doesn't exist. And looking more closely, it looks like the Cross project
has decided to shake things up and publish images to ghcr instead. So we
migrate everything over to that.

.github/workflows/ci.yml

@@ -75,6 +75,18 @@ jobs:
           os: ubuntu-latest
           rust: stable
           target: aarch64-unknown-linux-gnu
+        - build: stable-arm-gnueabihf
+          os: ubuntu-latest
+          rust: stable
+          target: armv7-unknown-linux-gnueabihf
+        - build: stable-arm-musleabihf
+          os: ubuntu-latest
+          rust: stable
+          target: armv7-unknown-linux-musleabihf
+        - build: stable-arm-musleabi
+          os: ubuntu-latest
+          rust: stable
+          target: armv7-unknown-linux-musleabi
         - build: stable-powerpc64
           os: ubuntu-latest
           rust: stable