Set MAKEFLAGS=-j and GTEST_BRIEF in .envrc

- Enable parallel builds by default (and allow using environment
  variables to override `make` variables)
  - Hopefully we can get rid of this once we have Meson
- Set `GTEST_BRIEF=1`
  - This only shows failed tests, instead of listing every test on its
    own line.

```
$ GTEST_BRIEF=1 make check
[==========] 328 tests from 15 test suites ran. (37 ms total)
[  PASSED  ] 328 tests.
```

Change-Id: Id8103a8f24a9681be2be87e1b4df6fd5fdd7e4fd

.envrc

@@ -1,3 +1,9 @@
 # shellcheck shell=bash
 source_env_if_exists .envrc.local
+# TODO: `use flake .#native-clangStdenvPackages` on macOS?
 use flake ".#${LIX_SHELL_VARIANT:-default}" "${LIX_SHELL_EXTRA_ARGS[@]}"
+export MAKEFLAGS="$MAKEFLAGS -e"
+if [[ -n "$NIX_BUILD_CORES" ]]; then
+    export MAKEFLAGS="$MAKEFLAGS -j $NIX_BUILD_CORES"
+fi
+export GTEST_BRIEF=1

.gitignore

@@ -157,3 +157,5 @@ result-*
 
 # ClangBuildAnalyzer output, see maintainers/buildtime_report.sh
 buildtime.bin
+
+.envrc.local