nix: Set PODMAN_BUILD in the default devShell

Otherwise, the value from the .config is used where it may be disabled
what leads to the following error:

$ nix develop -c make all
Redox podman build environment loaded
env -u RUSTUP_TOOLCHAIN -u CC -u TARGET cargo build --manifest-path Cargo.toml --release --locked
make: *** [mk/fstools.mk:47: build/fstools.tag] Error 127

flake.nix

@@ -118,6 +118,7 @@
                   LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath buildInputs;
                   SKIP_CHECK_TOOLS = "1";
                   FSTOOLS_IN_PODMAN = "1";
+                  PODMAN_BUILD = "1";
                   shellHook = ''
                     # Install required configuration
                     ${podmanSetupScript}