t6030: use $SHELL_PATH to invoke user's preferred shell instead of bare sh

Some platforms (IRIX, Solaris) provide an ancient /bin/sh which chokes on
modern shell syntax like $().  SHELL_PATH is provided to allow the user to
specify a working sh, let's use it here.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6030-bisect-porcelain.sh

@@ -701,7 +701,7 @@ test_expect_success 'bisect: demonstrate identification of damage boundary' "
 	git bisect reset &&
 	git checkout broken &&
 	git bisect start broken master --no-checkout &&
-	git bisect run sh -c '
+	git bisect run \"\$SHELL_PATH\" -c '
 		GOOD=\$(git for-each-ref \"--format=%(objectname)\" refs/bisect/good-*) &&
 		git rev-list --objects BISECT_HEAD --not \$GOOD >tmp.\$\$ &&
 		git pack-objects --stdout >/dev/null < tmp.\$\$