Try generating outputs in /tmp as a workaround for $GITHUB_WORKSPACE

(/__w/...) confusing asciidoctor-pdf, until there's a fix for
https://github.com/asciidoctor/asciidoctor-pdf/issues/1888

.github/workflows/CI.yml

@@ -22,13 +22,17 @@ jobs:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
 
+      # Unfortunately, asciidoctor-pdf gets pathname-specific errors
+      # building under the usual $GITHUB_WORKSPACE (/__w). As a workaround,
+      # generate the outputs in /tmp.
+
       - name: Build spec targets
         run: |
           cd adoc
-          make QUIET= html pdf
+          make OUTDIR=/tmp/out QUIET= html pdf
       - name: Archive generated files
         uses: actions/upload-artifact@v2
         with:
           name: spec-outputs
           path: |
-            adoc/out/
+            /tmp/out