build: Add support for when the build is sandboxed

When using flatpak-cargo-generator, the crates cannot be placed in the
build dir before the build happens, so we must change the CARGO_HOME env
variable.

meson.options

@@ -10,3 +10,11 @@ option(
   value: 'default',
   description: 'The build profile for Fractal. One of "default", "beta", "development" or "hack".'
 )
+option(
+  'sandboxed',
+  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'
+)

src/meson.build

@@ -39,7 +39,12 @@ else
   message('Building in release mode')
 endif
 
-cargo_env = [ 'CARGO_HOME=' + meson.project_build_root() / 'cargo-home' ]
+if get_option('sandboxed')
+  # 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
 
 custom_target(
   'cargo-build',