Fix unnecessary downloads of cached registry packages (#1571)

Closes #1544

Author: DelevoXDG

scarb/src/core/registry/client/cache.rs

@@ -145,7 +145,7 @@ impl<'c> RegistryClientCache<'c> {
             let tarball_name = package.tarball_name();
             let file = self
                 .dl_fs
-                .open_rw(&tarball_name, &tarball_name, self.config)?;
+                .open_ro(&tarball_name, &tarball_name, self.config)?;
             let checksum = self.get_record_maybe_uncached(package).await?.checksum;
             return Ok((file, checksum));
         }

scarb/tests/http_registry.rs

@@ -309,6 +309,15 @@ fn caching() {
         .success()
         .stdout_eq("");
 
+    Scarb::quick_snapbox()
+        .arg("fetch")
+        .env("SCARB_CACHE", cache_dir.path())
+        .current_dir(&t)
+        .timeout(Duration::from_secs(10))
+        .assert()
+        .success()
+        .stdout_eq("");
+
     let expected = expect![[r#"
         GET /api/v1/index/config.json
         accept: */*
@@ -365,6 +374,19 @@ fn caching() {
         304 Not Modified
         content-length: 0
         etag: ...
+
+        ###
+
+        GET /index/3/b/bar.json
+        accept: */*
+        accept-encoding: gzip, br, deflate
+        host: ...
+        if-none-match: ...
+        user-agent: ...
+
+        304 Not Modified
+        content-length: 0
+        etag: ...
     "#]];
     expected.assert_eq(&registry.logs());
 }