On some systems, such as the Apple M1 (ARM-based Macs),
dylibbundler is in $PATH, but not in a directory found by the limited
path that SCons uses for Command() invocations. This causes the SConf
test to succeed (because it uses the full $PATH), but the Command() to
fail (because it uses a reduced path). Fix this by using the user's
$PATH, if defined, for this Command() invocation.
Along the way, rework tool_bundle.py to use SCons' Node() objects in
place of direct path manipulation. This lets SCons produce better error
messages in some cases.
Reported-by: Kreeblah <https://github.com/dxx-rebirth/dxx-rebirth/pull/610>