ci: explicitly install rust toolchain

rustup 1.28.0 has made a breaking change: toolchains listed in
rust-toolchain.toml will no longer be automatically installed.

This explicitly installs the configured rust toolchain prior to build in
CI.

Author: cbgbt

.github/workflows/rust.yml

@@ -23,6 +23,7 @@ jobs:
       # Install `patch`, needed to build `krane-bundle`, and tools for linking against musl
       - run: sudo apt-get install -y patch musl-tools musl-dev
       - run: rustup target add x86_64-unknown-linux-musl
+      - run: rustup toolchain install
       - run: CARGO_BUILD_TARGET=x86_64-unknown-linux-musl make build
 
   cross-build: