Move nightly_version file to crate root

The nightly pinning is used by a bunch of different tools outside of
github actions, move the config file to the crate root.

Update the path in the justfile.

Done in preparation for fixing the git pre-commit hook.

.github/workflows/rust.yml

@@ -19,7 +19,7 @@ jobs:
         uses: actions/checkout@v4
       - name: Read nightly version
         id: read_toolchain
-        run: echo "nightly_version=$(cat .github/nightly-version)" >> $GITHUB_OUTPUT
+        run: echo "nightly_version=$(cat nightly-version)" >> $GITHUB_OUTPUT
       - name: Prepare tests
         id: get_matrix
         run: contrib/get_matrix.sh

justfile

@@ -11,11 +11,11 @@ check:
 
 # Lint everything.
 lint:
-  cargo +$(cat .github/nightly-version) clippy --workspace --all-targets --all-features -- --deny warnings
+  cargo +$(cat ./nightly-version) clippy --workspace --all-targets --all-features -- --deny warnings
 
 # Check the formatting
 format:
-  cargo +$(cat .github/nightly-version) fmt --all --check
+  cargo +$(cat ./nightly-version) fmt --all --check
 
 # Quick and dirty CI useful for pre-push checks.
 sane: lint