Depend on the NSIS plugin crates only where they are used

nsis-plugin-api compiles to nothing outside 32 bit Windows, but pulled
its dependencies in on all of Windows. Cargo warns that they are unused
when building for any other Windows target.

mullvad-nsis/vendor/nsis-plugin-api/Cargo.toml

@@ -26,7 +26,8 @@ autobenches = false
 name = "nsis_plugin_api"
 path = "src/lib.rs"
 
-[target."cfg(windows)".dependencies]
+# Same condition the crate itself is compiled under.
+[target.'cfg(all(target_arch = "x86", target_os = "windows"))'.dependencies]
 nsis-fn = { path = "../nsis-fn" }
 windows-sys = { workspace = true, features = [
   "Win32_Foundation",