commands: permissions - limit tests to Linux

Some `chmod` behavior is implementation defined (see: POSIX spec).
Since the 9P2000.L protocol is used, we test against GNU's `chmod`.
Trying to run these tests on Unix systems like Illumos, Darwin, et al.
will produce different results for the same symbolic expressions, as
they have different implementations / symbolic expression parsing rules.

Author: djdv

internal/commands/permission_test.go renamed to internal/commands/permission_linux_test.go

@@ -1,4 +1,4 @@
-//go:build unix
+//go:build linux
 
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