.gitea/workflows: enable unstable borrow checking

.gitea/workflows/build-and-test.yml

@@ -8,6 +8,7 @@ on:
 
 env:
   BINSTALL_DISABLE_TELEMETRY: 'true'
+  RUSTFLAGS: '-Zhigher-ranked-assumptions'
 
 jobs:
   build:
@@ -16,7 +17,7 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Install rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@nightly
 
       - name: Build
         run: cargo build --all-features --verbose --release
@@ -27,7 +28,7 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Install rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@nightly
         with:
             components: rustfmt, clippy
 
@@ -95,7 +96,7 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Install rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@nightly
 
       - name: Build docs
         run: cargo doc --all-features --document-private-items --release