fix(bats_test): use env var rather than symbolic link

[alexeagle]

This allows it to work on Windows as well.

Fixes red main build following #982
(note that the CI setup for this repo saves GHA quota by not running tests on Windows for all PRs, only when they come from a branch with 'windows' in the name)

Tested by using a little-known feature where Bazel can report the test case names:

$ bazel test lib/tests/bats:basic --test_summary=detailed
INFO: Analyzed target //lib/tests/bats:basic (3 packages loaded, 947 targets configured).
INFO: Found 1 test target...
Target //lib/tests/bats:basic up-to-date:
  bazel-bin/lib/tests/bats/basic_bats.sh
INFO: Elapsed time: 0.399s, Critical Path: 0.21s
INFO: 3 processes: 2 internal, 1 linux-sandbox.
INFO: Build completed successfully, 3 total actions
//lib/tests/bats:basic                                                   PASSED in 0.2s
    PASSED  basic.bats.assert_output() check for existence (0.0s)
Test cases: finished with 1 passing and 0 failing out of 1 test cases