pkgsStatic.bcachefs-tools: fix build

In pkgsStatic, there is no systemd.pc or udev.pc, so running make
without setting these variables will fail.  There's not really any
reason to only set them for installing anyway — in general it's best
to set variables like this for all phases to minimize the risk of a
mismatch between build and install behaviour.

pkgs/by-name/bc/bcachefs-tools/package.nix

@@ -70,6 +70,10 @@ stdenv.mkDerivation (finalAttrs: {
     "PREFIX=${placeholder "out"}"
     "VERSION=${finalAttrs.version}"
     "INITRAMFS_DIR=${placeholder "out"}/etc/initramfs-tools"
+
+    # Tries to install to the 'systemd-minimal' and 'udev' nix installation paths
+    "PKGCONFIG_SERVICEDIR=$(out)/lib/systemd/system"
+    "PKGCONFIG_UDEVDIR=$(out)/lib/udev"
   ] ++ lib.optional fuseSupport "BCACHEFS_FUSE=1";
 
   env = {
@@ -90,12 +94,6 @@ stdenv.mkDerivation (finalAttrs: {
   '';
   checkFlags = [ "BCACHEFS_TEST_USE_VALGRIND=no" ];
 
-  # Tries to install to the 'systemd-minimal' and 'udev' nix installation paths
-  installFlags = [
-    "PKGCONFIG_SERVICEDIR=$(out)/lib/systemd/system"
-    "PKGCONFIG_UDEVDIR=$(out)/lib/udev"
-  ];
-
   postInstall =
     ''
       substituteInPlace $out/libexec/bcachefsck_all \