Fix passing extra RUSTFLAGS

When the fix for rust-fuzz#141 is in effect if you pass extra RUSTFLAGS there is
no space separating them and the command line breaks. Fix that by adding
a space at the end of those flags, just like it's done in the common
string.

src/bin/cargo-afl.rs

@@ -194,8 +194,8 @@ where
         // work around https://github.com/rust-fuzz/afl.rs/issues/141 /
         // https://github.com/rust-lang/rust/issues/53945, can be removed once
         // those are fixed.
-        rustflags.push_str("-Clink-arg=-fuse-ld=gold");
-        rustdocflags.push_str("-Clink-arg=-fuse-ld=gold");
+        rustflags.push_str("-Clink-arg=-fuse-ld=gold ");
+        rustdocflags.push_str("-Clink-arg=-fuse-ld=gold ");
     }
 
     // add user provided flags