ci: add enable-profiling/ branch prefix

Created by: ggilmore

When pusing to a branch prefixed with enable-profiling/ (example: https://github.com/sourcegraph/sourcegraph/tree/enable-profiling/ggilmore)

• env time -v gets prefixed to every buildkite command
• a fair number of our scripts get more detailed timing information

Example output: https://buildkite.com/sourcegraph/sourcegraph/builds/58883#dd202d96-f6e3-458a-aacd-9d5715672ab9

 Command exited with non-zero status 1
  Command being timed: "./dev/check/all.sh"
  User time (seconds): 510.14
  System time (seconds): 171.30
  Percent of CPU this job got: 802%
  Elapsed (wall clock) time (h:mm:ss or m:ss): 1:24.95
  Average shared text size (kbytes): 0
  Average unshared data size (kbytes): 0
  Average stack size (kbytes): 0
  Average total size (kbytes): 0
  Maximum resident set size (kbytes): 12316844
  Average resident set size (kbytes): 0
  Major (requiring I/O) page faults: 14061
  Minor (reclaiming a frame) page faults: 10709094
  Voluntary context switches: 1946015
  Involuntary context switches: 325300
  Swaps: 0
  File system inputs: 2211272
  File system outputs: 3671368
  Socket messages sent: 0
  Socket messages received: 0
  Signals delivered: 0
  Page size (bytes): 4096
  Exit status: 1