Use installer args file for pip options

{{ cookiecutter.format }}/briefcase.toml

@@ -5,6 +5,7 @@ target_version = "0.3.20"
 [paths]
 app_path = "src/app"
 app_requirements_path = "requirements.txt"
+app_requirement_installer_args_path = "pip-options.txt"
 support_path = "support"
 {{ {
     "3.9": 'support_revision = "3.9.20+20241016"',

{{ cookiecutter.format }}/manifest.yml

@@ -75,12 +75,14 @@ modules:
       # command line compatible, and the output objects are compatible,
       # so we can override the CC build environment variable to force the
       # use of gcc.
-      - env CC="gcc -pthread" INSTALL_TARGET="/app/briefcase/app_packages" sh ./install_requirements.sh
+      - CC="gcc -pthread" /app/bin/python3 pip_install.py
     sources:
       - type: file
         path: requirements.txt
       - type: file
-        path: install_requirements.sh
+        path: pip-options.txt
+      - type: file
+        path: pip_install.py
   - name: app
     buildsystem: simple
     build-options:

{{ cookiecutter.format }}/pip_install.py

@@ -0,0 +1,33 @@
+from pathlib import Path
+import os
+import sys
+
+# Run pip install with default arguments + options read from the app requirement
+# installer args path.
+# execv is simpler than subprocess, as it avoids needing to manage the subprocess
+# or pipe in/out, which would add a lot of unnecessary complexity for no value.
+# This script only helps build the command, it does not need to check the output at all.
+
+os.execv(
+    sys.executable,
+    [
+        sys.executable,
+        "-m",
+        "pip",
+        "install",
+        "--no-cache-dir",
+        "-r",
+        "requirements.txt",
+        "--target",
+        "/app/briefcase/app_packages",
+    ]
+    + [
+        arg
+        for arg in (Path(__file__).parent / "pip-options.txt")
+        .read_text()
+        .split("\n")
+        # skip empty lines, including trailing newlines or empty default file
+        # (which contains a single newline character)
+        if arg and not arg.isspace()
+    ],
+)