Use pipx through py -%PIPX_PYTHON_VERSION%

Author: raulcd

ci/scripts/install_gcs_testbench.bat

@@ -24,8 +24,8 @@ set PIPX_FLAGS=--verbose
 py -%PIPX_PYTHON_VERSION% -m pip install -U pipx|| exit /B 1
 
 @REM Install GCS testbench %GCS_TESTBENCH_VERSION%
-pipx install --python py %PIPX_FLAGS% ^
+py -%PIPX_PYTHON_VERSION% -m pipx install --python py %PIPX_FLAGS% ^
         "https://github.com/googleapis/storage-testbench/archive/%GCS_TESTBENCH_VERSION%.tar.gz" ^
         || exit /B 1
 
-pipx list --verbose
+py -%PIPX_PYTHON_VERSION% -m pipx list --verbose