Super small improvement: Remove unnecessary mut (#3736)

## Issue Addressed

<!--Which issue # does this PR address?-->

Removed some unnecessary `mut`. 🙂 

<!--
## Proposed Changes

Please list or describe the changes introduced by this PR.
-->

<!--
## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.
-->

lcli/src/eth1_genesis.rs

@@ -13,7 +13,7 @@ use types::EthSpec;
 pub const ETH1_GENESIS_UPDATE_INTERVAL: Duration = Duration::from_millis(7_000);
 
 pub fn run<T: EthSpec>(
-    mut env: Environment<T>,
+    env: Environment<T>,
     testnet_dir: PathBuf,
     matches: &ArgMatches<'_>,
 ) -> Result<(), String> {