Move to Zig 0.13

[kristapsk]

Right now building with Zig 0.13.0 fails.

/home/user/git/ndg/build.zig:424:18: error: expected type '*const fn (*Build.Step, Progress.Node) anyerror!void', found '*const fn (*Build.Step, *Progress.Node) anyerror!void'
                .makeFn = make,
                ~^~~~~~~~~~~~~
/home/user/git/ndg/build.zig:424:18: note: pointer type child 'fn (*Build.Step, *Progress.Node) anyerror!void' cannot cast into pointer type child 'fn (*Build.Step, Progress.Node) anyerror!void'
/home/user/git/ndg/build.zig:424:18: note: parameter 1 '*Progress.Node' cannot cast into 'Progress.Node'
/home/user/zig-linux-x86_64-0.13.0/lib/std/Progress.zig:80:18: note: struct declared here
pub const Node = struct {
                 ^~~~~~
referenced by:
    build: /home/user/git/ndg/build.zig:16:36
    runBuild__anon_8825: /home/user/zig-linux-x86_64-0.13.0/lib/std/Build.zig:2116:27
    remaining reference traces hidden; use '-freference-trace' to see all reference traces
/home/user/git/ndg/lib/ini/build.zig:9:14: error: no field named 'path' in union 'Build.LazyPath'
            .path = "src/ini.zig",
             ^~~~
/home/user/zig-linux-x86_64-0.13.0/lib/std/Build.zig:2171:22: note: union declared here
pub const LazyPath = union(enum) {
                     ^~~~~

See 729af48.

Zig 0.13.0 release notes - https://ziglang.org/download/0.13.0/release-notes.html.