chore(ci): replace running `pre-commit` with just a formatting check

Contrary to what I naively thought, `pre-commit` in CI does not result
in commit messages being checked. Another action `pre-commit` was
doing was running `cargo check`, but this is essentially the same as
building without compilation, and we are testing by building in other
CI-workflows. So we are left with only a formatting check, which does
not need installing and running `pre-commit`.

.woodpecker/check_formatting.yaml

@@ -0,0 +1,11 @@
+steps:
+  check_formatting:
+    image: rust
+    environment:
+      RUST_BACKTRACE: 1
+      CARGO_TERM_COLOR: always
+    commands:
+      - rustup component add rustfmt
+      - cargo fmt --all -- --check
+    when:
+      event: [pull_request, push, tag]

.woodpecker/pre-commit.yaml

@@ -1,14 +0,0 @@
-steps:
-  pre-commit:
-    image: rust
-    environment:
-      RUST_BACKTRACE: 1
-      CARGO_TERM_COLOR: always
-    commands:
-      - apt-get -y update > /dev/null
-      - apt-get -y install pre-commit > /dev/null
-      - rustup default stable
-      - rustup component add rustfmt clippy
-      - pre-commit run --all-files
-    when:
-      event: [pull_request, push, tag]