ci(nvim-ts): build nvim against PR lib

Problem: The nvim-treesitter tests use a prebuilt Neovim, which means
changes to the library are not tested and parser generation is locked at
the ABI Neovim supports.

Solution: Build Neovim against the PR version of the library on Ubuntu
and macOS. On Windows, we still download binaries (because one does not
simply build Neovim on Windows...) so we still can check backward
compatibility.

.github/workflows/nvim_ts.yml

@@ -3,6 +3,7 @@ name: nvim-treesitter parser tests
 on:
   pull_request:
     paths:
+      - 'lib/**'
       - 'crates/cli/**'
       - 'crates/config/**'
       - 'crates/generate/**'
@@ -26,26 +27,43 @@ jobs:
     runs-on: ${{ matrix.os }}
     env:
       NVIM: ${{ matrix.os == 'windows-latest' && 'nvim-win64\\bin\\nvim.exe' || 'nvim' }}
+      NVIM_DIR: neovim
       NVIM_TS_DIR: nvim-treesitter
     steps:
       - uses: actions/checkout@v6
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - run: cargo build --profile optimize
+      - if: runner.os != 'Windows'
+        run: echo ${{ github.workspace }}/target/optimize >> "$GITHUB_PATH"
+      - if: runner.os == 'Windows'
+        run: echo ${{ github.workspace }}/target/optimize >> "$env:GITHUB_PATH"
 
-      - uses: actions/checkout@v6
+      - if: runner.os != 'Windows'
+        name: Clone Neovim
+        uses: actions/checkout@v6
+        with:
+          repository: neovim/neovim
+          path: ${{ env.NVIM_DIR }}
+
+      - if: runner.os != 'Windows'
+        name: Build Neovim
+        working-directory: ${{ env.NVIM_DIR }}
+        run: |
+          cmake -S cmake.deps -B .deps -G Ninja -D CMAKE_BUILD_TYPE=Release -D TREESITTER_URL=https://github.com/tree-sitter/tree-sitter/archive/${{ github.event.pull_request.head.sha }}.tar.gz -D DEPS_IGNORE_SHA=TRUE
+          cmake --build .deps --config Release
+          cmake -B build -G Ninja -D CMAKE_BUILD_TYPE=Release
+          cmake --build build --config Release
+          echo "VIMRUNTIME=${{ github.workspace }}/neovim/runtime" >> "$GITHUB_ENV"
+          echo ${{ github.workspace }}/neovim/build/bin >> "$GITHUB_PATH"
+
+      - name: Clone nvim-treesitter
+        uses: actions/checkout@v6
         with:
           repository: nvim-treesitter/nvim-treesitter
           path: ${{ env.NVIM_TS_DIR }}
-          ref: main
-
-      - if: runner.os != 'Windows'
-        run: echo ${{ github.workspace }}/target/optimize >> $GITHUB_PATH
 
       - if: runner.os == 'Windows'
-        run: echo ${{ github.workspace }}/target/optimize >> $env:GITHUB_PATH
-
-      - uses: actions-rust-lang/setup-rust-toolchain@v1
-      - run: cargo build --profile optimize
-
-      - name: Install and prepare Neovim
+        name: Install and prepare Neovim
         run: bash ./scripts/ci-install.sh
         working-directory: ${{ env.NVIM_TS_DIR }}