Preserve cpp-dotenv source during 'make clean' to improve build performance

- Remove cpp-dotenv source cleanup from 'make clean' in test/tap/tap/Makefile
- This prevents accidental removal of 213MB of extracted cpp-dotenv source
- Build targets force full rebuild anyway if libraries are missing
- Typical 'make clean' operation is now faster and preserves compiled artifacts

Similar optimization as test/deps:
- Regular development workflow preserves expensive-to-rebuild dependencies
- Only 'cleanall' removes everything including source directories
- Reduces repeated downloads and extraction of cpp-dotenv sources

This change complements the previous test/deps optimization, further
improving development workflow performance.

test/tap/tap/Makefile

@@ -118,6 +118,6 @@ clean:
 	find . -name '*.o' -delete || true
 	find . -name '*.so' -delete || true
 	find . -name '*.so.*' -delete || true
-	cd cpp-dotenv/static && rm -rf cpp-dotenv-*/ || true
-	cd cpp-dotenv/dynamic && rm -rf cpp-dotenv-*/ || true
+	# NOTE: cpp-dotenv source directories NOT cleaned here to preserve 213MB
+	# Build targets force full rebuild anyway if libs are missing