build-deps.py: add support for Python 3.15

We've added 3.15 to build-all-win, but `3.15.0` is not available on nuget yet, we need rc2 suffix for workflow to succeed.

win/installers.py

@@ -915,6 +915,13 @@ def install_qt6(
             require_exists("Qt6 host installation", path)
 
 
+def python_consider_rc(python_version: str) -> str:
+    # TODO: remove after Python 3.15 release.
+    if python_version == "3.15.0":
+        python_version += "-rc2"
+    return python_version
+
+
 def install_python(
     vs_cfg_vars: VsCfgResult,
     ifcos_install_python: bool,
@@ -927,6 +934,8 @@ def install_python(
         logger.info("IFCOS_INSTALL_PYTHON not 'TRUE', skipping installation of Python.")
         return None
 
+    python_version = python_consider_rc(python_version)
+
     if not vs_cfg_vars.is_vs_platform("ARM64") and not vs_cfg_vars.is_vs_platform("x64"):
         # nuget doesn't support providing architecture for packages.
         logger.error("Automatic installation of Python for x86 builds is not supported,")