[SYCL][UR] Fix fetch adapter source cache variable (#13934)

This caused issues in local builds because if the specific adapter
source is reset to the main UR checkout, the cache variable pointing to
the adapter specific checkout would stick around, and the adapter would
still be built from that older checkout which would eventually caused
build failures.

Author: npmiller

sycl/plugins/unified_runtime/CMakeLists.txt

@@ -75,6 +75,11 @@ if(SYCL_PI_UR_USE_FETCH_CONTENT)
     endif()
     if(repo STREQUAL UNIFIED_RUNTIME_REPO AND
         tag STREQUAL UNIFIED_RUNTIME_TAG)
+      # If the adapter sources are taken from the main checkout, reset the
+      # adapter specific source path.
+      string(TOUPPER ${name} NAME)
+      set(UR_ADAPTER_${NAME}_SOURCE_DIR ""
+        CACHE PATH "Path to external '${name}' adapter source dir" FORCE)
       return()
     endif()
     message(STATUS