Allow external zig in CMake

[RossComputerGuy]

What does this PR do?

Adds the ability to override the Zig command in use. This is useful for testing patches or odd systems like Asahi which rely on hacks or switching out to a locally built Zig.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

Configure cmake and set -DZIG_EXECUTABLE. Grep for --zig-lib-dir and saw the same path as zig env | jq .lib_dir says.

Example:

$ grep -r '\--zig-lib-dir'
CMakeFiles/bun-zig.dir/build.make:	cd /home/ross/bun-workspace/bun && /nix/store/kbi2k2h2q395ip75iq0zfra7p14cz0wx-zig-0.14.0-git+f70ec2f/bin/zig build obj --cache-dir /home/ross/bun-workspace/bun/build/cache/zig/local --global-cache-dir /home/ross/bun-workspace/bun/build/cache/zig/global --zig-lib-dir /nix/store/kbi2k2h2q395ip75iq0zfra7p14cz0wx-zig-0.14.0-git+f70ec2f/lib/zig --prefix /home/ross/bun-workspace/bun/build -Dobj_format=obj -Dtarget=aarch64-linux-gnu -Doptimize=ReleaseFast -Dcpu=native -Denable_logs=false -Dversion=1.2.3 -Dreported_nodejs_version=22.6.0 -Dcanary=1 -Dcodegen_path=/home/ross/bun-workspace/bun/build/codegen -Dcodegen_embed=true -Denable_asan=false --prominent-compile-errors -Dsha=9fd96c1be33fb612c7bc11ffb7cea565e868f879