devtools/jiri-test: retry sending welcome emails three times before failing.
devtools/jiri-test: retry sending welcome emails three times before failing.

This is to fix b/23945727

Change-Id: I8822e092a9deae6bee7db89b4963b6203d5c4ad5

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools