fix(rust): fix `cargo` completion when sysroot contains spaces (#10571)

When generating completions for Cargo, if the Rust sysroot
(i.e. `rustc +${${(z)$(rustup default)}[1]} --print sysroot`) contains
spaces, Cargo completions will not work because the spaces are not
escaped, thus passing two arguments to the "source" command instead of
one. The spaces need to be escaped for this to work.

plugins/rust/rust.plugin.zsh

@@ -27,5 +27,5 @@ fi
 rustup completions zsh >| "$ZSH_CACHE_DIR/completions/_rustup" &|
 cat >| "$ZSH_CACHE_DIR/completions/_cargo" <<'EOF'
 #compdef cargo
-source $(rustc +${${(z)$(rustup default)}[1]} --print sysroot)/share/zsh/site-functions/_cargo
+source "$(rustc +${${(z)$(rustup default)}[1]} --print sysroot)"/share/zsh/site-functions/_cargo
 EOF