feat(ci): build tfa and tftf in the job's workspace directory

This is useful when running locally - avoid polluting the source
directory with artefacts and allows more than 1 job to be built at the
same time.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I8e8e0ff4741d5454706fd96a55936d56c3a6d22e
diff --git a/run_config/fvp-trp.tftf b/run_config/fvp-trp.tftf
index 0793c8f..89a9597 100644
--- a/run_config/fvp-trp.tftf
+++ b/run_config/fvp-trp.tftf
@@ -8,7 +8,7 @@
 # Build TF-A with tf-a-tests and TRP
 post_tf_build() {
 	# Stash the TRP binary
-	archive_file "${tf_root}/build/fvp/${bin_mode}/rmm.bin"
+	archive_file "${tf_build_root}/fvp/${bin_mode}/rmm.bin"
 
 	build_fip BL33="$archive/tftf.bin" RMM="$archive/rmm.bin"
 }