[dexter] Fix failing regression tests

D109833 makes the flags `--builder` and `--binary` mutually exclusive, which
caused some regression tests to fail. Add a new substitution
`%dexter_regression_base` that doesn't include the `--builder`, `--cflags` or
`--ldflags` flags and use that for tests that use the `--binary` flag.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D112624