fix activation event when the workspace contains a build.zig

[Techatrix]

The glob pattern */build.zig means that the extension gets activated iff a subfolder contains a build.zig. Obviously we want to look in the root folder so the pattern build.zig should be used.

[Vexu]

Seems reasonable but maybe we should use onLanguage instead? https://code.visualstudio.com/api/references/activation-events#onLanguage

[Techatrix]

onLanguage:zig is already happening. It is automatically added when declaring the zig language. The problem with only relying on onLanguage is that the extension doesn't get activated before opening a .zig file. I often find myself in a new project with no open documents where I then use the integrated terminal to run zig build just to realize that the terminal doesn't have Zig in PATH because the extension hasn't started.