[Misc] Build Jtreg using JDK on aach64 runners

Summary: When the cache fails to be retrieved, the JDK used for the
build is forcibly set to the x64 JDK, which can cause problems in the
aarch64 runners.

Testing: CI pipline

Reviewers: lei.yul, sendaoYan

Issue: dragonwell-project#858

.github/actions/get-jtreg/action.yml

@@ -57,7 +57,12 @@ runs:
     - name: 'Build JTReg'
       run: |
         # Build JTReg and move files to the proper locations
-        bash make/build.sh --jdk "$JAVA_HOME_11_X64"
+        if [ -n "$JAVA_HOME_11_X64" ]; then
+            jdk=$JAVA_HOME_11_X64
+        else
+            jdk=$(dirname $(dirname $(readlink -f `which java`)))
+        fi
+        bash make/build.sh --jdk "$jdk"
         mkdir ../installed
         mv build/images/jtreg/* ../installed
       working-directory: jtreg/src