Add build.zig to paths in build.zig.zon

[ianprime0509]

Initially reported here: ziglang/zig#18282

If build.zig is not included in paths in build.zig.zon, the resulting package will not have that file included, leading to the package not being usable as intended, as seen through zig fetch --debug-hash:

$ zig fetch --debug-hash https://github.com/sam701/zig-cli/archive/e32f7454d690761ec2b4eab8600e0610d0d233bd.tar.gz
file: 54b645cc4a56212c6b1f97187aa6a129fd2eaebb9d8bea7cef04eb018eb7e208: build.zig.zon
file: fa9b337869d6c96162c0b84bd6889b38f445cffee40b6a9a8ffe0c06dd047d45: src/Printer.zig
file: c21c93b35cbd4fe8f7c59414652fb81f4cc06f7e0e0733b84f187c77a8443ca0: src/arg.zig
file: 1658e9acfe148c710d2754e42ab3e8a61d262bd46b8ba51510cda3ae3c0b98b6: src/command.zig
file: abce9fc3079e47c86ef9e1b0628b7ceca170b838c60857b98aa45c92844a706a: src/help.zig
file: 44478322ab927d0a57d8c2f442b76dca6b86e817c117d89ee3cfea6479ac4a17: src/main.zig
file: af69f2c9bd7f83454ff2368e986ccd4bc176f12628c3f10ce1e3fc927b1867c1: src/parser.zig
file: 9160cfb9b9d3ee6454dbfe34662a6ae471aae02b6ad1401ed48e40165b5a7c39: src/tests.zig
file: f0b90aebf22be930287b872faebf948494c34c7f1cb5bfcf51b962a9143a04c3: src/value_parser.zig
file: b6972db0a57450f4b70e4eeaa375ef683c6272b268bd8740b61062d40038eefe: src/value_ref.zig
12205d064a9bd4dac1ebce2cead5bc686ee85eb703b994495efd3b65115666555cd7

To fix this, I added build.zig to the paths field, and also updated the standalone example to point to this new version (which runs successfully for me on 0.12.0-dev.1828+225fe6ddb).

If this PR is merged using a merge commit, the update to the example's build.zig.zon should still be valid; otherwise (if any changes are made, e.g. using a squash commit or rebase), it will have to be updated again to reflect the final commit hash.

[sam701]

@ianprime0509 thank you for your contribution 👍