Looks like the same issue as Choco install hangs forever.
It has been suggested in that thread that adding filter_secrets: false can fix the issue. However, doing that seems quite dangerous when bad actors only need to submit a PR containing an env call to see all your secrets.
This issue is probably related to differences in Bash (4.3.48(1)) vs. Windows Bash (4.4.19(2)-release) and the secrets filtering not being compatible with Windows. Maybe somewhere in here: https://github.com/travis-ci/travis-build/tree/master/public/filter ??