ci(Makefile): Use system-agnostic path for bash.

Change `SHELL` variable in Makefile from `/bin/bash` to `/usr/bin/env bash`.

Author: sona-tau

Makefile

@@ -15,7 +15,7 @@
 #   You should have received a copy of the GNU General Public License
 #   along with this program.  If not, see http://www.gnu.org/licenses/.
 
-SHELL := /bin/bash
+SHELL := /usr/bin/env bash
 
 #Common prefixes