SL-18837: Try to install llsd in the Python used by py.exe

.github/workflows/build.yaml

@@ -32,10 +32,7 @@ jobs:
       # test-program log output at the specified level, but to display it only if
       # the individual test fails.
       LOGFAIL: DEBUG
-      # Force py.exe to select python3.9 rather than the newest install 3.11,
-      # since the 'pip3' command below seems to be associated with Python 3.9
-      PY_PYTHON: "3.9"
-      # but if unit tests fail to import llsd (i.e. wrong Python interpreter),
+      # if unit tests fail to import llsd (i.e. wrong Python interpreter),
       # make py.exe enumerate the possibilities and explain its choice
       PYLAUNCHER_DEBUG: "1"
     steps:
@@ -61,7 +58,13 @@ jobs:
 
       - name: Install windows dependencies
         if: runner.os == 'Windows'
-        run: choco install nsis-unicode
+        # Use 'py -m pip' because our autobuild.xml specifies py.exe as
+        # PYTHON_EXECUTABLE, and we've tried all kinds of tricks to make pip
+        # install the 'llsd' package into the Python interpreter selected by
+        # py.exe.
+        run: |
+          choco install nsis-unicode
+          py -m pip install autobuild llsd
 
       - name: Build
         id: build
@@ -69,7 +72,6 @@ jobs:
         env:
           RUNNER_OS: ${{ runner.os }}
         run: |
-          pip install llsd # Required by viewer tests
           autobuild configure
           autobuild build --no-configure