For some unclear reason, Android _now_ sometimes returns an
IsADirectory error, instead of PermissionDenied, when trying
to execute `.`.
Since this test really wants to test PermissionDenied, we try
to execute a file in the fixture instead, that doesn't have
exec permission.
Also, limit this test to Unix.
Fixes part of #7542.
.# Discussion
This commit adds support for a '-f'/'--file' option which reads "KEY=VALUE" lines from
a config (or ini) style text file and sets the corresponding environment key. This is
modeled after the same option in the `dotenv` and `godotenv` commands. Notably, this
commit does *not* add automatic loading of ".env" configuration files.
The environment variables set by reading the configuration file are set prior to any
unset (eg, `-u BAR`) or set (eg, `FOO=bar`) actions. Files are loaded in order with
later files overwriting any overlapping environment variables, then, unset actions (in
command line order) are executed, then, finally, set actions (in command line order)
are executed.
[1] [`dotenv`](https://github.com/bkeepers/dotenv)
[2] [`godotenv`](https://github.com/joho/godotenv)
