but the odd thing is that if I get the same error even with PKG_CONFIG_PATH pointing to the above:

[root@majortheobaldslapper:/tmp/nix-build-borgtui-fdcf13ba063a9e8516e5bc5f679af5504304819d.drv-3]# source build/env-vars

[root@majortheobaldslapper:/build/source]#  PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags fuse3 fuse3 >= 3.0.0
Package fuse3 was not found in the pkg-config search path.
Perhaps you should add the directory containing `fuse3.pc'
to the PKG_CONFIG_PATH environment variable
No package 'fuse3' found
Package fuse3 was not found in the pkg-config search path.
Perhaps you should add the directory containing `fuse3.pc'
to the PKG_CONFIG_PATH environment variable
No package 'fuse3' found
Package 3.0.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `3.0.0.pc'
to the PKG_CONFIG_PATH environment variable
No package '3.0.0' found

[root@majortheobaldslapper:/build/source]# echo $PKG_CONFIG
$PKG_CONFIG              $PKG_CONFIG_ALLOW_CROSS

[root@majortheobaldslapper:/build/source]# export PKG_CONFIG_PATH=/nix/store/c53z0p2yw9q2dfkxkvlpbz29g8n5isdc-fuse-3.16.2-dev/lib/pkgconfig/

[root@majortheobaldslapper:/build/source]#  PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags fuse3 fuse3 >= 3.0.0
Package 3.0.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `3.0.0.pc'
to the PKG_CONFIG_PATH environment variable
No package '3.0.0' found

[root@majortheobaldslapper:/build/source]# echo $PKG_CONFIG_PATH/
/nix/store/c53z0p2yw9q2dfkxkvlpbz29g8n5isdc-fuse-3.16.2-dev/lib/pkgconfig//

[root@majortheobaldslapper:/build/source]# ls -la $PKG_CONFIG_PATH/
total 12
dr-xr-xr-x 2 root root 4096 Jan  1  1970 .
dr-xr-xr-x 3 root root 4096 Jan  1  1970 ..
-r--r--r-- 2 root root  328 Jan  1  1970 fuse3.pc

[root@majortheobaldslapper:/build/source]#

yep, setting PKG_CONFIG_PATH would fix it as expected

[root@majortheobaldslapper:/build/source]#  PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags fuse3 fuse3>=3.0.0

[root@majortheobaldslapper:/build/source]# echo $?
0

[root@majortheobaldslapper:/build/source]# unset PKG_CONFIG_PATH

[root@majortheobaldslapper:/build/source]#  PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags fuse3 fuse3>=3.0.0
Package fuse3 was not found in the pkg-config search path.
Perhaps you should add the directory containing `fuse3.pc'
to the PKG_CONFIG_PATH environment variable
No package 'fuse3' found
Package fuse3 was not found in the pkg-config search path.
Perhaps you should add the directory containing `fuse3.pc'
to the PKG_CONFIG_PATH environment variable
No package 'fuse3' found

In reply to @staberinde:matrix.org

Currently I have

borg-tui = pkgs.rustPlatform.buildRustPackage (finalAttrs: rec {
    pname = "borgtui";
    version = "fdcf13ba063a9e8516e5bc5f679af5504304819d";
    src = pkgs.fetchFromGitHub {
      owner = "dpbriggs";
      repo = pname;
      rev = version;
      sha256 = "7UbwW53HTM4oTbDnNruwjtF0KrnhoiCYXWl0CbXfMSk=";
    };
    nativeBuildInputs = [
      pkgs.pkg-config
    ];
    bulldInputs = [
      pkgs.fuse3
    ];
    cargoHash = "sha256-7F87unpr3OoY9SFBS2iwKhk2khYKH5w5xaENHUwnqC0=";
  });