os: add flatpak detection

src/os/flatpak.zig

@@ -0,0 +1,9 @@
+const std = @import("std");
+const builtin = @import("builtin");
+
+/// Returns true if we're running in a Flatpak environment.
+pub fn isFlatpak() bool {
+    // If we're not on Linux then we'll make this comptime false.
+    if (comptime builtin.os.tag != .linux) return false;
+    return if (std.fs.accessAbsolute("/.flatpak-info", .{})) true else |_| false;
+}