check-non-portable-shell: suggest alternative for `VAR=val shell-func`

check-non-portable-shell: suggest alternative for `VAR=val shell-func`

Most problems reported by check-non-portable-shell are accompanied by
advice suggesting how the test author can repair the problem. For
instance:

    error: egrep/fgrep obsolescent (use grep -E/-F)

However, when one-shot variable assignment is detected when calling a
shell function (i.e. `VAR=val shell-func`), the problem is reported, but
no advice is given. The lack of advice is particularly egregious since
neither the problem nor the workaround are likely well-known by
newcomers to the project writing tests for the first time. Address this
shortcoming by recommending the use of `test_env` which is tailor made
for this specific use-case.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>