github-ci: update debian 9 test to use known Rust version

Choose Rust 1.39.0 as a known version of Rust that we build against
to see in CI if we build with this version, but fail with latest.

This is to cover a similar test from Travis CI.

.github/workflows/builds.yml

@@ -22,6 +22,9 @@ env:
   # Apt sometimes likes to ask for user input, this will prevent that.
   DEBIAN_FRONTEND: "noninteractive"
 
+  # A known good Rust version we should test against.
+  RUST_VERSION_KNOWN: "1.37.0"
+
 jobs:
 
   prep:
@@ -1004,7 +1007,7 @@ jobs:
                 zlib1g \
                 zlib1g-dev
       - name: Install Rust
-        run: curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.34.2 -y
+        run: curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain $RUST_VERSION_KNOWN -y
       - run: echo "::add-path::$HOME/.cargo/bin"
       - name: Install cbindgen
         run: cargo install --force --debug --version 0.14.1 cbindgen