Add configurable binary mirror host for restricted networks

[krmorse]

Fixes #287

Summary

• Adds support for redirecting the prebuilt binary download to an internal mirror via SKIA_CANVAS_BINARY_HOST env var or --skia_canvas_binary_host_mirror npm config flag
• Falls back to the default GitHub Releases URL if neither is set
• Documents the feature in the Installation section of the README

Test plan

• Install with SKIA_CANVAS_BINARY_HOST set to a mirror and verify the binary is fetched from that host
• Install with --skia_canvas_binary_host_mirror flag and verify the binary is fetched from that host
• Install without either set and verify default behavior is unchanged

🤖 Generated with Claude Code

[krmorse]

apparently npm 11/12 is changing the way these flags work too, so worth some additional thought there...