Build failed due to: spawn_tests.gpr:7:06: imported project file "spawn" not found

Hello! I'm trying to build the library from sources in an MSYS2 environment. I noticed that make all will fail on running gprbuild for gnat/tests/spawn_tests.gpr with the following error:

# make all
gprbuild -p -j0 -P gnat/spawn.gpr
Setup
   [mkdir]        object directory for project Spawn
   [mkdir]        library directory for project Spawn
Compile
   [Ada]          spawn.ads
   [Ada]          spawn-windows_api.adb
   [Ada]          spawn-string_vectors.ads
   [Ada]          spawn-process_listeners.ads
   [Ada]          spawn-processes.adb
   [Ada]          spawn-processes-monitor_loop.adb
   [Ada]          spawn-posix.ads
   [Ada]          spawn-polls.ads
   [Ada]          spawn-internal-windows.adb
   [Ada]          spawn-environments.adb
   [Ada]          spawn-common.adb
   [Ada]          spawn-internal__windows.adb
   [Ada]          spawn-environments-internal__windows.adb
   [Ada]          spawn-internal-monitor__windows.adb
Build Libraries
   [gprlib]       spawn.lexch
   [archive]      libspawn.a
   [index]        libspawn.a
gprbuild -p -j0 -P gnat/tests/spawn_tests.gpr -XSPAWN_LIBRARY_TYPE=static
spawn_tests.gpr:7:06: imported project file "spawn" not found
spawn_tests.gpr:15:04: warning: no when others for this case construction
spawn_tests.gpr:15:09: unknown package or project "Spawn"
spawn_tests.gpr:27:29: "Spawn" is not an imported or extended project
gprbuild: "gnat/tests/spawn_tests.gpr" processing failed
make: *** [Makefile:22: all] Error 5

I do not have pre-installed Spawn library in the environment, therefore spawn_tests.gpr cannot resolve the with "spawn"; clause which is supposed to refer to a project file in the parent directory.

The same error occurs in GitHub Actions CI, see the pipeline: https://github.com/AdaCore/spawn/actions/runs/7029769102

Start with gnat/tests/spawn_tests.gpr, especially its with "spawn" clause, and compare it with the parent project gnat/spawn.gpr. Reproduce the failure with make all in MSYS2 or the linked GitHub Actions run. Done means the test project resolves the library project without a pre-installed Spawn library and make all passes.