Getting started: update install.sh to run with bash not sh

[raptorswing]

The script uses [[ ]] syntax for conditionals, which is a bash feature not supported in vanilla sh.

Tested on Ubuntu 25.10. The current install script errors with:

./install.sh: 16: [[: not found
./install.sh: 50: [[: not found
./install.sh: 69: [[: not found
#=O#-    #       #                                                                                                                                                                                                                    curl: (22) The requested URL returned error: 404

On this system, /bin/sh is symlinked to dash.
On some systems where /bin/sh is symlinked to bash, the current script would work fine.

This change will make the script work on more systems. A future change could revise the script to be POSIX shell compliant if there's a big need for this to run on systems that don't have bash.

Complementary PR to change the shebang in install.sh to say /bin/bash for clarity: DioxusLabs/dioxus#4965