Dub does not cache single-file builds with DFLAGS

[CyberShadow]

$ DFLAGS=-unittest dub test.d
(compiles test.d...)
(runs test.d)

$ DFLAGS=-unittest dub test.d
(compiles test.d... again)
(runs test.d)

For some reason this only applies when DFLAGS is set. Without it, Dub caches the executable as expected.

See also #2672.