build: Rename sandboxed option to sandboxed-build

To make its purpose clearer

meson.options

@@ -11,7 +11,7 @@ option(
   description: 'The build profile for Fractal. One of "default", "beta", "development" or "hack".'
 )
 option(
-  'sandboxed',
+  'sandboxed-build',
   type : 'boolean',
   value : false,
   description: 'Whether the build happens in a sandbox.' +

src/meson.build

@@ -39,7 +39,7 @@ else
   message('Building in release mode')
 endif
 
-if get_option('sandboxed')
+if get_option('sandboxed-build')
   # This is the path used by flatpak-cargo-generator in flatpak-builder-tools
   cargo_env = [ 'CARGO_HOME=' + meson.project_source_root() / 'cargo' ]
 else