Refactor pip install commands in mfc.rb

Author: sbryngelson

Formula/mfc.rb

@@ -29,13 +29,11 @@ def install
     # Create Python virtual environment inside libexec (inside Cellar for proper bottling)
     venv = libexec/"venv"
     system Formula["python@3.12"].opt_bin/"python3.12", "-m", "venv", venv
-    
     system venv/"bin/pip", "install", "--upgrade",
            "pip", "setuptools", "wheel",
            "setuptools-scm",
            "hatchling", "hatch-vcs",
            "editables"
-
     # Install Cantera from PyPI using pre-built wheel (complex package, doesn't need custom flags)
     # Cantera has CMake compatibility issues when building from source with newer CMake versions
     # Match the version constraint from toolchain/pyproject.toml
@@ -47,8 +45,9 @@ def install
     # Keep toolchain in buildpath for now - mfc.sh needs it there
     #
     # MFC's toolchain uses VCS-derived versioning (via Hatch/hatch-vcs) and Homebrew builds from
-    # GitHub release tarballs without a .git directory. Scope pretend-version env vars tightly
-    # to avoid polluting subsequent pip installs.
+    # GitHub release tarballs without a .git directory. Use --no-build-isolation so the build
+    # backend can see our environment variables, and set SETUPTOOLS_SCM_PRETEND_VERSION which
+    # hatch-vcs respects when VCS metadata is unavailable.
     pretend_env = {
       "SETUPTOOLS_SCM_PRETEND_VERSION_FOR_MFC" => version.to_s,
       "SETUPTOOLS_SCM_PRETEND_VERSION_FOR_mfc" => version.to_s,
@@ -61,7 +60,7 @@ def install
     end
 
     begin
-      system venv/"bin/pip", "install", "-e", buildpath/"toolchain"
+      system venv/"bin/pip", "install", "--no-build-isolation", "-e", buildpath/"toolchain"
     ensure
       pretend_env.each_key do |k|
         if saved_env[k].nil?
@@ -79,7 +78,12 @@ def install
     # Now build MFC with pre-configured venv
     # Set VIRTUAL_ENV so mfc.sh uses existing venv instead of creating new one
     ENV["VIRTUAL_ENV"] = venv
-
+    
+    # Also set pretend-version env vars for mfc.sh in case it tries to reinstall toolchain
+    ENV["SETUPTOOLS_SCM_PRETEND_VERSION_FOR_MFC"] = version.to_s
+    ENV["SETUPTOOLS_SCM_PRETEND_VERSION_FOR_mfc"] = version.to_s
+    ENV["SETUPTOOLS_SCM_PRETEND_VERSION"] = version.to_s
+    
     # Build MFC using pre-configured venv
     # Must run from buildpath (MFC root directory) where toolchain/ exists
     Dir.chdir(buildpath) do
@@ -349,4 +353,4 @@ def caveats
     assert_path_exists testpath_case/"silo_hdf5"
     assert_predicate testpath_case/"silo_hdf5", :directory?
   end
-end
+end