ci: build artifacts workflow to accept a tag

Running from the tag on the workflow itself doesn't necessarily work here because it will use an
older process, which doesn't help if you want to build binaries that were not part of that old
process.

.github/workflows/build-release-artifacts.yml

@@ -8,6 +8,10 @@ on:
         type: string
         required: true
         default: main
+      tag:
+        description: The tag to build.
+        type: string
+        required: false
 
 # Copied from `release.yml`
 # During the build step, the env variable has to be manually sent to the containers for cross platform builds.
@@ -43,7 +47,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          ref: ${{ inputs.branch }}
+          ref: ${{ inputs.tag || inputs.branch }}
       - uses: dtolnay/rust-toolchain@stable
       # cargo-binstall will try and use pre-built binaries if they are available and also speeds up
       # installing `cross`