Avoid duplicating PATH

[mnijhuis-tos]

When using the singleMachine batch system, environment contains the saved contents of os.environ.

This code then effectively concatenates the original PATH saved in environment to the PATH in os.environ, which is equal. In the end, os.environ thus gets extended with its original contents.

In my case, my PATH was quite long. After toil concatenated it several times to os.environ, os.environ contained 7 times the original PATH. Later, an "Argument list too long" error occured while executing a simple stat call.

If environment["PATH"] already contains os.environ["PATH"] (which is also true of os.environ["PATH"] is empty or if it equals environment["PATH]), the concatenation is not needed, so we can skip it and avoid the "Argument list too long" error.

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