github: Don't assume GitHub workspace path on Windows

The "Test USD" job on Windows requires PATH and PYTHONPATH
to be updated to point to various USD build directories.
These were hard-coded to paths on the D:\ since that's
where we believed the workspace was located. However, we've
recently observed runs where the workspace was on C:\ instead,
causing tests to fail.

To avoid this issue, we now use the github.workspace variable
instead of assuming a specific workspace path.

(Internal change: 2368631)

Author: sunyab

.github/workflows/buildusd.yml

@@ -188,8 +188,8 @@ jobs:
       - name: Test USD
         working-directory: ./USDgen/build/OpenUSD
         run: |
-          call set PATH=D:\a\OpenUSD\OpenUSD\USDinst\bin;D:\a\OpenUSD\OpenUSD\USDinst\lib;D:\a\OpenUSD\OpenUSD\USDinst\share\usd\examples\plugin;D:\a\OpenUSD\OpenUSD\USDinst\plugin\usd;%PATH%
-          call set PYTHONPATH=D:\a\OpenUSD\OpenUSD\USDinst\lib\python;%PYTHONPATH%
+          call set PATH=${{ github.workspace }}\USDinst\bin;${{ github.workspace }}\USDinst\lib;${{ github.workspace }}\USDinst\share\usd\examples\plugin;${{ github.workspace }}\USDinst\plugin\usd;%PATH%
+          call set PYTHONPATH=${{ github.workspace }}\USDinst\lib\python;%PYTHONPATH%
           # Internal ticket USD-8035
           ctest -V -C Release --exclude-regex "TfPathUtils|testExecGeomXformable_Perf_Large"
         shell: cmd