Pass SwiftOverlay_DIR to ExperimentalObservation

Author: Steelskin

.github/workflows/swift-toolchain.yml

@@ -2694,6 +2694,7 @@ jobs:
                 -G Ninja `
                 -S ${{ github.workspace }}/SourceCache/swift/Runtimes/Supplemental/Observation `
                 -D SwiftCore_DIR=${{ github.workspace }}/BinaryCache/ExperimentalRuntime/cmake/SwiftCore `
+                -D SwiftOverlay_DIR=${{ github.workspace }}/BinaryCache/ExperimentalOverlay/cmake/SwiftOverlay
       - name: Build Experimental Observation
         if: matrix.os != 'Android' || inputs.build_android
         run: |