fix: installation bug

[jeevithakannan2]

Running the script as root will cause issue as the fvm binary resides in /root

readlink /usr/local/bin/fvm 
/root/.fvm_flutter/bin/fvm

This PR proposes the following changes

• Check if the script is running as root.

• Check for the correct escalation tool like sudo or doas.

• Use escalation inside the script as symlinking to /usr requires escalated privileges

• Resolves [BUG] Failed to create symlink. #796

[vercel]

@jeevithakannan2 is attempting to deploy a commit to the FlutterTools Team on Vercel.

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[silverark]

I've just hit this issue on Ubuntu 24. When installing, it downloads and unpacks FVM, but can't create the symbolic link as it doesn't have permissions to write to /usr/local/bin/

I can confirm this script fixes the issue, by asking for permission as it installs. I came here to patch it myself but glad someone beat me to it.