gpu-dawn: add NO_ENSURE_SUBMODULES=true env var for development

If you're working on changes to source files in `gpu-dawn/libs/dawn/`,
it's annoying to have the build system constantly reset your revision to
the desired one.

`NO_ENSURE_SUBMODULES=true` will now disable this behavior.

Helps #86

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu-dawn/build.zig

@@ -177,6 +177,9 @@ fn linkFromSource(b: *Builder, step: *std.build.LibExeObjStep, options: Options)
 }
 
 fn ensureSubmodules(allocator: std.mem.Allocator) !void {
+    if (std.process.getEnvVarOwned(allocator, "NO_ENSURE_SUBMODULES")) |no_ensure_submodules| {
+        if (std.mem.eql(u8, no_ensure_submodules, "true")) return;
+    } else |_| {}
     const child = try std.ChildProcess.init(&.{ "git", "submodule", "update", "--init", "--recursive" }, allocator);
     child.cwd = thisDir();
     child.stderr = std.io.getStdErr();