Disable precompiled headers in 'nix develop'

They're still enabled in regular builds though.

flake.nix

@@ -136,7 +136,7 @@
 
           enableParallelBuilding = true;
 
-          makeFlags = "profiledir=$(out)/etc/profile.d";
+          makeFlags = "profiledir=$(out)/etc/profile.d PRECOMPILE_HEADERS=1";
 
           doCheck = true;
 
@@ -334,9 +334,6 @@
             # syntax-check generated dot files, it still requires some
             # fonts.  So provide those.
             FONTCONFIG_FILE = texFunctions.fontsConf;
-
-            # To test building without precompiled headers.
-            makeFlagsArray = [ "PRECOMPILE_HEADERS=0" ];
           };
 
         # System tests.

mk/precompiled-headers.mk

@@ -1,4 +1,4 @@
-PRECOMPILE_HEADERS ?= 1
+PRECOMPILE_HEADERS ?= 0
 
 print-var-help += \
   echo "  PRECOMPILE_HEADERS ($(PRECOMPILE_HEADERS)): Whether to use precompiled headers to speed up the build";