P7: Instance of Diskstore doesn't understand #isWritable

[Bajger]

Bug description
In Exercism Pharo track is OSSubprocess project used. However it fails on P7 (backward compatibility is handled there).
During initialization of OSVMProcess class, is OldStandardFileStream>>open:forWrite: sent, which causes a DNU message.

To Reproduce
Steps to reproduce the behavior:

1. Download P7 official release
2. Run MC command:

Metacello new
 	baseline: 'OSSubprocess';
 	repository: 'github://pharo-contributions/OSSubprocess:v1.3.0/repository';
	load.

4. See error: "Instance of FileSystem did not understand #isWritable"

Expected behavior
Package is successfully loaded

Screenshots

Version information:

• OS: macOS Monterey (12.2.1)
• Pharo Version P7-64bit stable

Expected development cost

1. Add P7 compatibility package that would be loaded prior OSSubprocess package.
2. Change implementation of OldStandardFileStream>>open:forWrite:
   to use fileName asFileReference isWriteable instead of: FileSystem disk isWritable not that causes DNU.
3. Add a change to compatibility package.

Additional context
P7 is used by some of the users of Exercism Pharo track.