Vendor deps.

vendor/target-lexicon/.github/actions/install-rust/README.md

@@ -0,0 +1,18 @@
+# install-rust
+
+A small github action to install `rustup` and a Rust toolchain. This is
+generally expressed inline, but it was repeated enough in this repository it
+seemed worthwhile to extract.
+
+Some gotchas:
+
+* Can't `--self-update` on Windows due to permission errors (a bug in Github
+  Actions)
+* `rustup` isn't installed on macOS (a bug in Github Actions)
+
+When the above are fixed we should delete this action and just use this inline:
+
+```yml
+- run: rustup update $toolchain && rustup default $toolchain
+  shell: bash
+```