Merge #880: ci: free bloat for MSRV job

dfe17ba ci: free bloat for MSRV job (Nick Johnson)

Pull request description:

  The MSRV job is running out of memory in CI due to a recent change to Github's underlying runner. Sounds like this will be resolved at some point in the future, actions/runner-images#13315, but until then remove the extra bloat.
  
  Copying over from rust-bitcoin: rust-bitcoin/rust-bitcoin#5310


ACKs for top commit:
  apoelstra:
    ACK dfe17ba; successfully ran local tests


Tree-SHA512: 3f62bcd4a3e323a53d5710a30f120afaacab165d1b010d90bbf923f9990231b2147afae3079e65e36e6013aad8ac83a60c11d15d198fd1610cf7af48c970db55

Author: apoelstra

.github/workflows/rust.yml

@@ -85,6 +85,12 @@ jobs:
           repository: rust-bitcoin/rust-bitcoin-maintainer-tools
           ref: c3324024ced9bb1eb854397686919c3ff7d97e1e
           path: maintainer-tools
+      - name: "Free disk space"
+        uses: endersonmenezes/free-disk-space@v3
+        with:
+          remove_android: true
+          remove_dotnet: true
+          remove_haskell: true
       - name: "Select toolchain"
         uses: dtolnay/rust-toolchain@stable
         with: