build: Remove sandboxed-build meson option

Since we only need this to use vendored dependencies for Flathub,
we can just modify the Cargo config file in Flathub's flatpak manifest
for that.

meson.options

@@ -10,14 +10,6 @@ option(
   value: 'default',
   description: 'The build profile for Fractal. One of "default", "beta", "development" or "hack".'
 )
-option(
-  'sandboxed-build',
-  type : 'boolean',
-  value : false,
-  description: 'Whether the build happens in a sandbox.' +
-               'When that is the case, cargo will not be able to download the dependencies during' +
-               'the build so they are assumed to be in `{meson.project_source_root()}/cargo`.'
-)
 option(
   'disable-glycin-sandbox',
   type : 'boolean',

src/meson.build

@@ -40,12 +40,7 @@ else
   message('Building in release mode')
 endif
 
-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
-  cargo_env = [ 'CARGO_HOME=' + meson.project_build_root() / 'cargo-home' ]
-endif
+cargo_env = [ 'CARGO_HOME=' + meson.project_build_root() / 'cargo-home' ]
 
 custom_target(
   'cargo-build',