don't include derivation name in temporary build directories

With the migration to /nix/var/nix/builds we now have failing builds
when the derivation name is too long.
This change removes the derivation name from the temporary build to have
a predictable prefix length:

Also see: NixOS/infra#764
for context.

Author: Mic92

src/libstore/unix/build/derivation-builder.cc

@@ -706,7 +706,7 @@ void DerivationBuilderImpl::startBuilder()
 
     /* Create a temporary directory where the build will take
        place. */
-    topTmpDir = createTempDir(buildDir, "nix-build-" + std::string(drvPath.name()), 0700);
+    topTmpDir = createTempDir(buildDir, "nix", 0700);
     setBuildTmpDir();
     assert(!tmpDir.empty());