ASP logging functions misformat BUILD file names as build target names

[chrisnovakovic]

MWE:

## test/BUILD

fatal("whoops")

$ plz build //test
Building [0/1, 0.0s]:
  CPU use:   0.0%  I/O:   0.0%  Mem use:   0.0%
11:16:06.791 CRITICAL: //test/BUILD: whoops

The critical log message should say test/BUILD, not //test/BUILD: it's a file name, not a build target name.