small fixes to compile script

[philomates]

Hello, today I was checking out grasp and ran into two small hiccups with the compile script.

escape bash variable during printing

before

$ ./script/compile
Please set 

after

$ ./script/compile
Please set $GRAALVM_HOME

isolate native-image requirement because it asks for running things as sudo

This change is a question of taste, so feel free to ask that I omit it.

before

$ ./script/compile
:direct-linking true
grasp.native
Error: Insufficient privileges for administration of the GraalVM installation. You need to become "root" user in order to perform administrative tasks on GraalVM. 
NOTE: depending on your operating system, you may need to use OS tools to install or uninstall GraalVM components.

This makes me want to run sudo ./script/compile which feels sketchy. Instead, one can adapt the script to check if native-image is installed and if not, ask that you install it. This installation should be the only thing that needs sudo.

so after that:

$ script/compile
Please install graal's "native-image" with
"$GRAALVM_HOME/bin/gu" install native-image

$ "$GRAALVM_HOME/bin/gu" install native-image
Error: Insufficient privileges for administration of the GraalVM installation. You need to become "root" user in order to perform administrative tasks on GraalVM. 
NOTE: depending on your operating system, you may need to use OS tools to install or uninstall GraalVM components.

$ sudo /usr/lib/jvm/java-8-graalvm/bin/gu install native-image
...

[borkdude]

Thanks!