builder: fix msys2

msys2 does not `/bin/command` or `/usr/bin/command`, check for these and
fall back to `/bin/sh -c "command $*"`.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

tools/builder/core.sh

@@ -2909,7 +2909,13 @@ gpg() {
 }
 
 command() {
-    /bin/command "$@"
+    if [ -x /bin/command ]; then
+        /bin/command "$@"
+    elif [ -x /usr/bin/command ]; then
+        /usr/bin/command "$@"
+    else
+        /bin/sh -c "command $*"
+    fi
 }
 
 fully_resolve_link() {