Cache CI output only when pushing to main. (#80)

## Content

Cache CI output only when pushing to main. We don't want to pollute
cache with builds of experimental / untrusted code. This also speeds up
CI.

## Pre-submit checklist

- Branch
    - [x] Tests are provided (if possible)
    - [x] Commit sequence broadly makes sense
    - [x] Key commits have useful messages
- PR
    - [x] No clippy warnings in the CI
    - [x] Self-reviewed the diff
    - [x] Useful pull request description
    - [x] Reviewer requested
- Documentation
    - [x] Update README file (if relevant)
    - [x] Update documentation website (if relevant)

## Issue(s)

TODO

.github/workflows/rust-ci.yml

@@ -5,6 +5,9 @@ on:
     branches: ["main"]
   pull_request:
 
+env:
+  push_main: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
+
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -55,6 +58,7 @@ jobs:
         run: cargo doc --no-deps --verbose
 
       - name: Cache Cargo registry
+        if: ${{ env.push_main == true }}
         uses: actions/cache@v3
         with:
           path: ~/.cargo/registry
@@ -63,6 +67,7 @@ jobs:
             ${{ runner.os }}-cargo-registry-
 
       - name: Cache Cargo build
+        if: ${{ env.push_main == true }}
         uses: actions/cache@v3
         with:
           path: target