ci: do not store Rust cache from PRs

<https://github.com/swatinem/rust-cache>
caches only dependencies, and
dependencies don't change most of the time,
so PRs store the same cache
as already stored
by the main branch commit PRs are based on.

Hash of Cargo.{toml,lock} is part of the
cache key, so for dependency updating PRs
the cache key is new. Such PRs
rebuild everything from scratch,
which is a separate problem.
Storing such cache from PR is however
not useful because most of the time
dependency updating PR is going
to be rebased again before merging
as Dependabot PRs are opened in a batch
and then merged one by one
while Dependabot rebases remaining PRs.

.github/workflows/ci.yml

@@ -41,6 +41,8 @@ jobs:
         shell: bash
       - name: Cache rust cargo artifacts
         uses: swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
       - name: Run rustfmt
         run: cargo fmt --all -- --check
       - name: Run clippy
@@ -92,6 +94,8 @@ jobs:
           persist-credentials: false
       - name: Cache rust cargo artifacts
         uses: swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
       - name: Rustdoc
         run: cargo doc --document-private-items --no-deps
 
@@ -135,6 +139,8 @@ jobs:
 
       - name: Cache rust cargo artifacts
         uses: swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
 
       - name: Install nextest
         uses: taiki-e/install-action@5f57d6cb7cd20b14a8a27f522884c4bc8a187458
@@ -169,6 +175,8 @@ jobs:
 
       - name: Cache rust cargo artifacts
         uses: swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
 
       - name: Build C library
         run: cargo build -p deltachat_ffi
@@ -195,6 +203,8 @@ jobs:
 
       - name: Cache rust cargo artifacts
         uses: swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
 
       - name: Build deltachat-rpc-server
         run: cargo build -p deltachat-rpc-server