Make OUTPUT_DIRECTORY in ProdDoxyfile configurable

Bug: skia:9514
Change-Id: I628c2a59302e22c8d4c0973def0a3a11f7f882b7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260817
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>

docker/cloudbuild.yaml

@@ -27,6 +27,8 @@ steps:
 
   - name: 'gcr.io/skia-public/doxygen:testing-slim'
     dir: './tools/doxygen'
+    env:
+      - 'OUTPUT_DIRECTORY=/workspace/__doxygen_staging'
     args: ['doxygen', 'ProdDoxyfile']
     timeout: 600s
 
@@ -150,4 +152,4 @@ images:
   - 'gcr.io/$PROJECT_ID/skia-wasm-release:prod'
 timeout: 7200s
 options:
-  machineType: 'N1_HIGHCPU_32'
+  machineType: 'N1_HIGHCPU_32'

tools/doxygen/ProdDoxyfile

@@ -2,4 +2,4 @@
 
 @INCLUDE = Doxyfile
 
-OUTPUT_DIRECTORY       = /workspace/__doxygen_staging
+OUTPUT_DIRECTORY = $(OUTPUT_DIRECTORY)