CI: Update Emscripten Docker image to 3.1.51

[kleisauke]

Emscripten does not guarantee ABI compatibility across its versions, so we need ensure that this version is kept in-sync with the wasm-vips project.

Note that there is *no* ABI compatibility guarantee between versions - the ABI
may change, so that we can keep improving and optimizing it. The compiler will
automatically invalidate system caches when the version number updates, so that
libc etc. are rebuilt for you. You should also rebuild object files and
libraries in your project when you upgrade emscripten.

From: https://github.com/emscripten-core/emscripten/blob/main/ChangeLog.md?plain=1#L7-L11

Depends on: lovell/sharp-libvips#212.

[lovell]

Thanks Kleis, new sharp release with libvips 8.15.1 on its way shortly.

[lovell]

I added an emscripten version check via commit 3bc31a8