ci: Test the build on an old nightly

Add a CI job that runs `cargo xtask build` with an old
nightly. Currently the oldest nightly that works is 2022-04-18. There's
nothing particularly magic about that version; we can require a newer
version when we deem it appropriate. Having this CI job just makes it
easy for us to identify in advance when a PR is going to require a newer
nightly, while still allowing other CI jobs to use latest nightly.

This change is related to (but not a fix for):
#510

Author: nicholasbishop

.github/workflows/msrv_toolchain.toml

@@ -0,0 +1,4 @@
+[toolchain]
+# Oldest nightly that currently works with `cargo xtask build`.
+channel = "nightly-2022-04-18"
+components = ["rust-src"]

.github/workflows/rust.yml

@@ -157,3 +157,21 @@ jobs:
 
       - name: Build
         run: cargo xtask build
+
+  # Run the build with our current nightly MSRV (specified in
+  # ./msrv_toolchain.toml). This serves to check that we don't
+  # accidentally start relying on a new feature without intending
+  # to. Having a test for this makes it easier for us to be intentional
+  # about making changes that require a newer version.
+  build_msrv:
+    name: Check that the build works on our nightly MSRV
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v2
+
+    - name: Set toolchain
+      run: cp .github/workflows/msrv_toolchain.toml rust-toolchain.toml
+
+    - name: Build
+      run: cargo xtask build