fix: pypi environment issues

[saikonen]

fixes logic introduced in #1595 which incorrectly evaluates any @pypi environment as disabled.

tested with

from metaflow import step, FlowSpec, pypi, pypi_base, conda, conda_base

class EnvTest(FlowSpec):
    @pypi
    @step
    def start(self):
        print("Starting 👋")
        import sys
        print(sys.executable)
        print(sys.path)
        self.next(self.end)

    @step
    def end(self):
        print("Done! 🏁")


if __name__ == "__main__":
    EnvTest()

which should include the micromamba environment and Python executable (before this PR it would only point to the host Python)

Resolves #1605