CI: Fix commit field in $:/build

bin/build-site.sh

@@ -32,10 +32,14 @@ if [  -z "$TW5_BUILD_DETAILS" ]; then
     TW5_BUILD_DETAILS="$(git symbolic-ref --short HEAD)-$(git rev-parse HEAD) from $(git remote get-url origin)"
 fi
 
-TW5_BUILD_COMMIT="$(git rev-parse HEAD)"
-
 echo "Using TW5_BUILD_DETAILS as [$TW5_BUILD_DETAILS]"
 
+if [  -z "$TW5_BUILD_COMMIT" ]; then
+	TW5_BUILD_COMMIT="$(git rev-parse HEAD)"
+fi
+
+echo "Using TW5_BUILD_COMMIT as [$TW5_BUILD_COMMIT]"
+
 # Set up the build output directory
 
 if [  -z "$TW5_BUILD_OUTPUT" ]; then
@@ -76,7 +80,7 @@ echo "<a href='./plugins/tiddlywiki/tahoelafs/index.html'>Moved to http://tiddly
 
 # Put the build details into a .tid file so that it can be included in each build (deleted at the end of this script)
 
-echo -e -n "title: $:/build\ncommit: $TW_BUILD_COMMIT\n\n$TW5_BUILD_DETAILS\n" > $TW5_BUILD_OUTPUT/build.tid
+echo -e -n "title: $:/build\ncommit: $TW5_BUILD_COMMIT\n\n$TW5_BUILD_DETAILS\n" > $TW5_BUILD_OUTPUT/build.tid
 
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