SL-18837: Install autobuild from source repo, not PyPI.

Bug in autobuild 3.9.2? The TC viewer build can successfully build
llphysicsextensions/autobuild-tpv.xml, but the previous GH actions build
failed: "no configuration for Release found" despite -c Tpv. The difference
seems to be that we used 'pip3 install autobuild' from PyPI, whereas the TC
buildscripts setup clones secondlife/autobuild branch v3 and installs from
that repo.

.github/workflows/build.yaml

@@ -28,6 +28,7 @@ jobs:
     env:
       AUTOBUILD_ADDRSIZE: ${{ matrix.addrsize }}
       AUTOBUILD_BUILD_ID: ${{ github.run_id }}
+      autobuild_checkout: ${{ github.workspace}}/.autobuild
       AUTOBUILD_CONFIGURATION: ${{ matrix.configuration }}
       # authorizes fetching private constituent packages
       AUTOBUILD_GITHUB_TOKEN: ${{ secrets.SHARED_AUTOBUILD_GITHUB_TOKEN }}
@@ -66,6 +67,19 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Checkout autobuild
+        uses: actions/checkout@v3
+        # Bug in autobuild 3.9.2? The TC viewer build can successfully build
+        # llphysicsextensions/autobuild-tpv.xml, but the previous GH actions
+        # build failed: "no configuration for Release found" despite -c Tpv.
+        # The difference seems to be that we used 'pip3 install autobuild'
+        # from PyPI, whereas the TC buildscripts setup clones
+        # secondlife/autobuild branch v3 and installs from that repo.
+        with:
+          repository: secondlife/autobuild
+          ref: v3
+          path: .autobuild
+
       - name: Checkout build variables
         uses: actions/checkout@v3
         with:
@@ -80,7 +94,10 @@ jobs:
           path: .master-message-template
 
       - name: Install autobuild
-        run: pip3 install autobuild llsd
+        shell: bash
+        run: |
+          pip3 install "$autobuild_checkout"
+          pip3 install llsd
 
       - name: Cache autobuild packages
         uses: actions/cache@v3