Use posix sh function definition syntax in build script

build.sh

@@ -28,7 +28,7 @@ EOF
     exit 1
 fi
 
-function doTest {
+doTest() {
     if [[ $# -eq 0 ]]; then
         exec stack test
     fi
@@ -48,7 +48,7 @@ function doTest {
     esac
 }
 
-function doBench {
+doBench() {
     exec stack bench :bench --benchmark-arguments "$*"
 }