Try to set PKG_CONFIG_PATH in the environment. (#45)

This demonstrates how we can avoid setting it later in each step that needs it.  Also we generate llvm.pc into a subdirectory to avoid finding it just because it's in the current working directory.

.github/workflows/test.yml

@@ -105,17 +105,21 @@ jobs:
       with:
         path: Swifty-LLVM
 
-    - name: Create LLVM pkgconfig file
-      run: |
+    - name: Create LLVM pkgconfig file and make it findable
+      run: >-
         set -ex -o pipefail
-        PATH="${{ github.workspace }}/${{ env.package_basename }}/bin:$PATH" Swifty-LLVM/Tools/make-pkgconfig.sh llvm.pc
+
+        mkdir pkg-config
+
+        PATH="${{ github.workspace }}/${{ env.package_basename }}/bin:$PATH"
+        Swifty-LLVM/Tools/make-pkgconfig.sh pkg-config/llvm.pc
+
+        echo "PKG_CONFIG_PATH=${GITHUB_WORKSPACE//\\//}/pkg-config" >> $GITHUB_ENV
       shell: bash
 
     - name: Test
       run: swift test -v ${{ matrix.swift_test_options }} -c ${{ matrix.configuration }}
       working-directory: Swifty-LLVM
-      env:
-        PKG_CONFIG_PATH: ${{ github.workspace }}
 
     - name: Export Coverage
       if: ${{ contains(matrix.swift_test_options, '--enable-code-coverage') }}