Delete uncompressed genesis states (#2092)

## Issue Addressed

Replaces #2091

## Proposed Changes

* Delete the uncompressed genesis states from `eth2_network_config` after they were merged accidentally in #2029.
* Tweak the build script to not overwrite `genesis.ssz` on every build, which caused spurious rebuilds.

.gitignore

@@ -7,3 +7,4 @@ flamegraph.svg
 perf.data*
 *.tar.gz
 /bin
+genesis.ssz

common/eth2_network_config/build.rs

@@ -1,5 +1,4 @@
-//! Downloads a network configuration from Github.
-
+//! Extracts zipped genesis states on first run.
 use eth2_config::{
     altona, mainnet, medalla, pyrmont, spadina, toledo, Eth2NetArchiveAndDirectory,
     GENESIS_FILE_NAME,
@@ -31,7 +30,16 @@ fn main() {
 
 /// Uncompress the network configs archive into a network configs folder.
 fn uncompress_state(network: &Eth2NetArchiveAndDirectory<'static>) -> Result<(), String> {
+    let genesis_ssz_path = network.dir().join(GENESIS_FILE_NAME);
+
+    // Take care to not overwrite the genesis.ssz if it already exists, as that causes
+    // spurious rebuilds.
+    if genesis_ssz_path.exists() {
+        return Ok(());
+    }
+
     if network.genesis_is_known {
+        // Extract genesis state from genesis.ssz.zip
         let archive_path = network.genesis_state_archive();
         let archive_file = File::open(&archive_path)
             .map_err(|e| format!("Failed to open archive file {:?}: {:?}", archive_path, e))?;
@@ -45,18 +53,14 @@ fn uncompress_state(network: &Eth2NetArchiveAndDirectory<'static>) -> Result<(),
                 GENESIS_FILE_NAME, e
             )
         })?;
-        let path = network.dir().join(GENESIS_FILE_NAME);
-        let mut outfile = File::create(&path)
-            .map_err(|e| format!("Error while creating file {:?}: {}", path, e))?;
+        let mut outfile = File::create(&genesis_ssz_path)
+            .map_err(|e| format!("Error while creating file {:?}: {}", genesis_ssz_path, e))?;
         io::copy(&mut file, &mut outfile)
-            .map_err(|e| format!("Error writing file {:?}: {}", path, e))?;
+            .map_err(|e| format!("Error writing file {:?}: {}", genesis_ssz_path, e))?;
     } else {
         // Create empty genesis.ssz if genesis is unknown
-        let genesis_file = network.dir().join(GENESIS_FILE_NAME);
-        if !genesis_file.exists() {
-            File::create(genesis_file)
-                .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
-        }
+        File::create(genesis_ssz_path)
+            .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
     }
 
     Ok(())