Add missing TrimRight in URLPrefix template variable

This caused the SSO server to generate bad form links, which caused a
302, preventing POST requests to succeed.
4 jobs for master in 2 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #6355
test

00:00:26

 
  Build Src
passed #6356
build:src

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  Build Pkg
passed #6357
build:pkg

00:00:46

 
  Upload
passed #6358
upload:pkg

00:00:31