munix: make the default command overridable via env

This is convenient for app launch wrappers that would want to set their
own default command but still allow overriding it

munix

@@ -3,6 +3,7 @@ SCRIPT_PATH=$(dirname $(realpath -s $0))
 MUVM_PATH=$(dirname $(which muvm))
 PASST_PATH=$(dirname $(which passt))
 HOST_OPENGL_DRIVER=/run/opengl-driver
+: "${MICROVM_DEFAULT_COMMAND:=bash}"
 MICROVM_CLOSURE=
 MICROVM_COMMAND=()
 MICROVM_UID=1337
@@ -71,7 +72,7 @@ fi
 MICROVM_CLOSURE=$(realpath "$MICROVM_CLOSURE")
 
 if [ ${#MICROVM_COMMAND[@]} -eq 0 ]; then
-  MICROVM_COMMAND=("bash")
+  MICROVM_COMMAND=("$MICROVM_DEFAULT_COMMAND")
 fi
 
 if [ "$GPU" -eq 1 ]; then