ci: install cargo-rbmt in GitHub Actions

Author: KyrylR

.github/actions/setup-rbmt/action.yml

@@ -0,0 +1,18 @@
+name: Setup RBMT
+description: Install cargo-rbmt and pinned Rust toolchains.
+
+runs:
+  using: "composite"
+  steps:
+    - name: Install cargo-rbmt
+      shell: bash
+      run: |
+        cargo install \
+          --git https://github.com/rust-bitcoin/rust-bitcoin-maintainer-tools \
+          --rev "$(cat "${GITHUB_WORKSPACE}/rbmt-version")" \
+          cargo-rbmt \
+          --locked
+
+    - name: Install Rust toolchains
+      shell: bash
+      run: cargo rbmt toolchains