GHA: attempt to track swift-atomics revision

There is now a swift-atomics dependency in the toolchain. The repo
manifest pins the revision and we resolve it as necessary. This just
makes it explicit so that we can be certain that it is available when
needed and it gets serialised into the stable manifest.

Author: compnerd

.github/workflows/swift-toolchain.yml

@@ -68,6 +68,7 @@ jobs:
       sourcekit_lsp_revision: ${{ steps.context.outputs.sourcekit_lsp_revision }}
       swift_argument_parser_revision: ${{ steps.context.outputs.swift_argument_parser_revision }}
       swift_asn1_revision: ${{ steps.context.outputs.swift_asn1_revision }}
+      swift_atomics_revision: ${{ steps.context.outputs.swift_atomics_revision }}
       swift_certificates_revision: ${{ steps.context.outputs.swift_certificates_revision }}
       swift_cmark_revision: ${{ steps.context.outputs.swift_cmark_revision }}
       swift_collections_revision: ${{ steps.context.outputs.swift_collections_revision }}
@@ -126,6 +127,7 @@ jobs:
           swift_revision=refs/tags/${{ inputs.swift_tag }}
           swift_argument_parser_revision=refs/tags/1.2.2
           swift_asn1_revision=refs/tags/0.7.0
+          swift_atomics_revision=refs/tags/1.2.0
           swift_certificates_revision=refs/tags/0.1.0
           swift_cmark_revision=refs/tags/${{ inputs.swift_tag }}
           swift_collections_revision=refs/tags/1.0.4