ci: fix sccache C++ caching on Windows for Python builds

[cpsievert]

Summary

• Fix sccache not caching C/C++ compilations (DuckDB) on Windows by adding ilammy/msvc-dev-cmd and setting CC/CXX='sccache cl'
• Remove path filters so Python tests run on all repo changes (justified by the speed improvements below)

Problem

Windows Python builds take ~30 minutes vs ~7 min on other platforms. Investigation of sccache stats revealed that Windows gets zero C/C++ cache hits while Ubuntu gets ~350:

	Ubuntu	Windows (before)
sccache C/C++ hits	243	0
sccache C/C++ misses	106	0
Build time	~5 min	~27 min

The root cause: the cc crate's MSVC cl.exe detection (via find_msvc_tools) bypasses the RUSTC_WRAPPER-based sccache fallback that works automatically on Linux/macOS. The libduckdb-sys C++ build (~350 source files) was compiling from scratch every run.

Fix

Two additions to the workflow:

1. ilammy/msvc-dev-cmd — Runs vcvarsall.bat to put cl.exe on PATH
2. CC/CXX='sccache cl' (Windows only) — Makes the cc crate's env_tool parser recognize sccache as a known wrapper and cl as the compiler

Results

After the sccache cache was seeded (first run: 350 C/C++ misses, 0 write errors), the next run showed:

	Before fix	After fix
C/C++ cache hits	0	325 (93%)
Rust cache hits	232 (53%)	241 (55%)
Build time	~25 min	~12.5 min
Total job time	~30 min	~15 min

The Rust sccache is still warming up (55% hit rate), so further improvement is expected on subsequent runs as more Rust compilation results are cached.

Path filter removal

Previously the Python workflow only ran on changes to ggsql-python/ and the workflow file itself. Since the Python bindings depend on the core Rust library, changes elsewhere (parser, writer, reader) could break them silently. With Windows builds cut in half (~15 min and improving), running Python tests on all changes is now practical.

Test plan

• Verify sccache sees C/C++ compilations on Windows (350 misses on seeding run)
• Verify 0 sccache write errors (cache successfully stored)
• Verify C/C++ cache hits on subsequent run (325 hits, 93% hit rate)
• Verify Windows build time reduced (~30 min → ~15 min)
• Verify Linux/macOS builds are unaffected

🤖 Generated with Claude Code