buildtypes/meson.sh.in: Work around environment variable setting

The function `start_cmd()` in the bee shell runs "{@}" causing the
setting of the environment variable to be executed, which is of course
not found.

```
function start_cmd() {
    print_info "${COLOR_CYAN}${@}"
    "${@}"
}
```

Work around it, by setting the environment variable `DESTDIR` outside.

buildtypes/meson.sh.in

@@ -50,6 +50,6 @@ bee_build() {
 #### bee_install() ############################################################
 
 bee_install() {
-    start_cmd DESTDIR=${D} ninja -C ${B} install "${@}"
+    DESTDIR=${D} start_cmd ninja -C ${B} install "${@}"
 }