Allow TARGET_REPO to be parametrized

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

Author: bensze01

vars/mbedtls-pr-test-Jenkinsfile

@@ -49,4 +49,4 @@
 
 /* main job */
 library identifier: 'mbedtls-test@master', retriever: legacySCM(scm)
-mbedtls.run_pr_job('tls', false, env.MBED_TLS_BRANCH, env.RUN_TF_PSA_CRYPTO_ALL_SH == 'true' ? env.TF_PSA_CRYPTO_BRANCH : '')
+mbedtls.run_pr_job(env.TARGET_REPO ?: 'tls', false, env.MBED_TLS_BRANCH, env.RUN_TF_PSA_CRYPTO_ALL_SH == 'true' ? env.TF_PSA_CRYPTO_BRANCH : '')