scm/url: fix crash if no extractor is installed

[rhubert]

Fix a crash on a system without 'unzip':

Traceback (most recent call last):
  File "/bob/pym/bob/scripts.py", line 146, in catchErrors
    ret = fun(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^
  File "/bob/pym/bob/scripts.py", line 254, in cmd
    ret = cmd(args.args, bobRoot)
          ^^^^^^^^^^^^^^^^^^^^^^^
  File "/bob/pym/bob/scripts.py", line 29, in __develop
    doDevelop(*args, **kwargs)
  File "/bob/pym/bob/cmds/build/build.py", line 410, in doDevelop
    commonBuildDevelop(parser, argv, bobRoot, True)
  File "/bob/pym/bob/cmds/build/build.py", line 345, in commonBuildDevelop
    builder.cook([ExecutableStep.fromStep(b, LazyIR) for b in backlog],
  File "/bob/pym/bob/builder.py", line 916, in cook
    raise self.__buildErrors[0]
  File "/bob/pym/bob/builder.py", line 811, in __taskWrapper
    ret = await coro()
          ^^^^^^^^^^^^
  File "/bob/pym/bob/builder.py", line 970, in _cookStep
    await self._cookCheckoutStep(step, depth)
  File "/bob/pym/bob/builder.py", line 1173, in _cookCheckoutStep
    await self._runShell(checkoutStep, "checkout", a)
  File "/bob/pym/bob/builder.py", line 737, in _runShell
    ret = await invoker.executeStep(mode, cleanWorkspace)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/bob/pym/bob/invoker.py", line 379, in executeStep
    await scm.invoke(self)
  File "/bob/pym/bob/scm/url.py", line 371, in invoke
    executor.fail("No suitable extractor found!")
    ^^^^^^^^
NameError: name 'executor' is not defined

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0fd7793) 87.92% compared to head (2b1cabb) 87.93%.

Files	Patch %	Lines
pym/bob/scm/url.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #538   +/-   ##
=======================================
  Coverage   87.92%   87.93%           
=======================================
  Files          46       46           
  Lines       14509    14509           
=======================================
+ Hits        12757    12758    +1     
+ Misses       1752     1751    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jkloetzke]

Thanks!