Limit the number of simultaneous jobs

We might be trying to overdo it with the number of CPU cores available.
If we limit the jobs, it might speed up the build.

.circleci/config.yml

@@ -13,6 +13,8 @@ jobs:
       - run:
           name: Compile
           command: nix-shell --run 'make sdist test -j2'
+          environment:
+            CABAL_BUILD_FLAGS: --jobs=2
       - save_cache:
           key: v2-nix-{{ checksum "package.yaml" }}
           name: Caching nix

Makefile

@@ -1,5 +1,6 @@
 CABAL ?= cabal
 CABAL_FLAGS ?=
+CABAL_BUILD_FLAGS ?=
 DIST ?= dist
 GHCID ?= ghcid
 GHCID_FLAGS ?= --ghc-options=-fno-code
@@ -22,7 +23,7 @@ $(CONFIGURE): $(CABAL_FILE)
 
 .PHONY: build
 build $(DOC_TEST): $(CONFIGURE) default.nix
-	$(CABAL) $(CABAL_FLAGS) build
+	$(CABAL) $(CABAL_FLAGS) build $(CABAL_BUILD_FLAGS)
 
 .PHONY: check
 check: $(CABAL_FILE)