Add support for adding to cache key

When using this action in multiple jobs in the same workflow, the
generated cache key is the same for all of them. This means that all
apart from the first job are unable to save the cache, and subsequent
runs might restore the wrong cache.

The `Swatinem/rust-cache` action which we use for caching has a `key`
input which it puts in its cache key. (It doesn't override the key, just
adds to it.) Providing this as an input here will allow us to generate a
unique cache key for each job.

README.md

@@ -56,6 +56,7 @@ Afterward, the `components` and `target` specified via inputs are installed in a
 | `cache`            | Automatically configure Rust cache (using [`Swatinem/rust-cache`])                     | true          |
 | `cache-workspaces` | Propagates the value to [`Swatinem/rust-cache`]                                        |               |
 | `cache-on-failure` | Propagates the value to [`Swatinem/rust-cache`]                                        | true          |
+| `key`              | Propagates the value to [`Swatinem/rust-cache`]                                        |               |
 | `matcher`          | Enable problem matcher to surface build messages and formatting issues                 | true          |
 | `rustflags`        | Set the value of `RUSTFLAGS` (set to empty string to avoid overwriting existing flags) | "-D warnings" |
 

action.yml

@@ -32,6 +32,9 @@ inputs:
     description: "Also cache on workflow failures"
     default: "true"
     required: false
+  key:
+    description: "An additional cache key that is added alongside the automatic `job`-based cache key and can be used to further differentiate jobs."
+    required: false
   matcher:
     description: "Enable the Rust problem matcher"
     required: false
@@ -181,3 +184,4 @@ runs:
       with:
         workspaces: ${{inputs.cache-workspaces}}
         cache-on-failure: ${{inputs.cache-on-failure}}
+        key: ${{inputs.key}}