[ci] Removed parallelism

nix/frama-c.nix

@@ -71,7 +71,7 @@ stdenv.mkDerivation rec {
 
   buildPhase = ''
     make config.sed
-    dune build --display short @install
+    dune build -j2 --display short @install
   '';
 
   installFlags = [

nix/main-tests.nix

@@ -35,7 +35,7 @@ stdenv.mkDerivation rec {
     make ptests/ptests.exe
     make ptests/wtests.exe
     dune exec --root ptests -- frama-c-ptests tests
-    dune build --display short @tests/ptests
+    dune build -j1 --display short @tests/ptests
   '';
 
   # No installation required