azure: tests: re-run flaky proxy tests

The proxy tests regularly fail in our CI environment. Unfortunately,
this is expected due to the network layer. Thus, let's re-try the proxy
tests up to five times in case they fail.

azure-pipelines/test.sh

@@ -216,7 +216,9 @@ if [ -z "$SKIP_PROXY_TESTS" ]; then
 	export GITTEST_REMOTE_PROXY_HOST="localhost:8090"
 	export GITTEST_REMOTE_PROXY_USER="foo"
 	export GITTEST_REMOTE_PROXY_PASS="bar"
+	export GITTEST_FLAKY_RETRY=5
 	run_test proxy
+	unset GITTEST_FLAKY_RETRY
 	unset GITTEST_REMOTE_PROXY_HOST
 	unset GITTEST_REMOTE_PROXY_USER
 	unset GITTEST_REMOTE_PROXY_PASS