gst-inspect: Disable colors when piped

This follows what git and systemd tools would do.
6 jobs for fix-inspect-pager-colors in 27 minutes and 4 seconds (queued for 2 minutes and 23 seconds)
Status Job ID Name Coverage
  Preparation
passed #67967
gst indent

00:00:14

passed #67966
manifest

00:00:18

 
  Build
passed #67968
build fedora x86_64

00:01:55

 
  Test
passed #67969
check fedora

00:16:48

 
  Full Builds
passed #67970
build android arm64 api28

00:04:01

passed #67971
build cerbero fedora x86_64

00:07:57