[SV 59230] Ensure environment variables are exportable

[SV 59230] Ensure environment variables are exportable

When checking for invalid environment variable names we searched the
entire name string instead of just the first LENGTH chars; this could
cause us to incorrectly decide the variable was not exportable.

Dmitry Goncharov <dgoncharov@users.sf.net> found this bug and
provided a test case and sample fix: I used the test but chose a
slightly different fix.

* src/variable.c (define_variable_in_set): check the variable name
not the input string.
* tests/scripts/features/targetvars: Ensure environment variable
values are exported.