Fix restoring GOBIN

`GOBIN` wasn't being remembered when calling `in`, thus wasn't being restored when `out` is called.

In the current branch, running the tests with a non-empty `GOBIN` fails:

```
inkel@lookfar ~/dev/gvp (master) ↩
↪ GOBIN=/tmp/bin make test
cd test && ./run_all_tests.sh
>> Now running all tests
>> Current test: init_in_and_out_test.sh
>> Local GOPATH set.
>> Reverted to system GOPATH.
test #8 "echo " failed:
	expected "/tmp/bin"
	got nothing
1 of 9 examples tests failed in 0s.
>> All Done
>> Build status: failing
make: *** [test] Error 1
```

The reason is that `gvp in` isn't exporting `GVP_OLD_GOBIN`, and `gvp out` restores `GOBIN` to the contents of the undefined variable `GVP_OLD_BIN_PATH`. The tests weren't failing because normally `GOBIN` is empty. With this change, now all tests passes:

```
inkel@lookfar ~/dev/gvp (restore-gobin) ↩
↪ GOBIN=/tmp/bin make test
cd test && ./run_all_tests.sh
>> Now running all tests
>> Current test: init_in_and_out_test.sh
>> Local GOPATH set.
>> Reverted to system GOPATH.
>> All 9 examples tests passed in 1s.
>> All Done
>> Build status: passing
inkel@lookfar ~/dev/gvp (restore-gobin) ↩
↪ make test
cd test && ./run_all_tests.sh
>> Now running all tests
>> Current test: init_in_and_out_test.sh
>> Local GOPATH set.
>> Reverted to system GOPATH.
>> All 9 examples tests passed in 0s.
>> All Done
>> Build status: passing
```