…ler.

GN set the Ninja `restat` argument to 1 by default. Which means that it
notes the modification timestamp when writing the contents. Its man page
states "Scripts can improve build performance by taking care not to
change the timstamp of the output file(s) if the contents have not
changed.".

Modifying the compiler will now still re-compile shaders. But if the
shaders have not changed, downstream edges will be culled.

This can be observed in the Ninja progress update after this change.
After editing any file in the compiler, Ninja start out with
`(2/866) LINK ./impellerc`. But as it culls edges, it skips tasks and
ends with `(454/455) ACTION //flutter/...`. More than half the edges are
culled. More importantly, the unit-tests edges are culled as well. For
workflows where only the changed tests are run, this saves a ton of test
time in addition to build time.