[gdb/testsuite] Fix gdb.base/local-env.exp on remote host

With host/target board local-remote-host-native.exp and other remote host
configurations, and test-case gdb.base/local-env.exp I get:
...
(gdb) show environment^M
  ...
(gdb) FAIL: $exp: show environment displayed variable
...

The test attempt to detect variable GDB_TEST_ENV_VAR in the environment, which
has been set with setenv.

This doesn't work with remote host, so declare the test unsupported.  Likewise
in gdb.base/environ.exp.

Tested on x86_64-linux with make-check-all.sh.