Remove mold suggestion in .envrc.example (#688)

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### What

It's totally reasonable that a user would just copy `.envrc.example` to
`.envrc` and then get going, however this means we try and use `mold`,
which we do not install in the Nix flake, and does not work on MacOS.
Let's make the default path as conservative and likely to work as
possible.

### How

Remove `mold` advice from `.envrc.example`, make `CARGO_INCREMENTAL`
flag opt in.

Author: danieljharvey

.envrc.example

@@ -7,7 +7,4 @@
 use flake
 
 # enable incremental compilation
-export CARGO_INCREMENTAL='true'
-
-# use `mold` as your linker, for faster linking (only on Linux)
-export RUSTFLAGS='-C link-arg=-fuse-ld=mold'
+# export CARGO_INCREMENTAL='true'