Build: Add update script

This allows to group different update commands together and to raise
awareness of the update task.

The `grep '[^0-9]_[0-9] flake.lock'` is needed to check if multiple
imports exists for the same input as nix will name them 'nixpkgs_1'
'nixpkgs_2' and so on. Having multiple inputs for the same thing just
increases the needed storage space, if no other inputs are set to
follow, but can break a flake's evaluation because of a partial update
e.g., nixpkgs follows our version, but we leave rust-overlay unfollowed.
This example would result in a newer cargo version (rust-overlay) getting
combined with old packages (nixpkgs), which introduces the
aforementioned partial update.

1 files changed, 5 insertions, 0 deletions

diff --git a/update b/update
new file mode 100755
index 0000000..6e76b0e
--- /dev/null
+++ b/update
@@ -0,0 +1,5 @@
+#!/bin/sh
+# shellcheck disable=SC2086
+nix flake update
+
+! grep '[^0-9]_[0-9]' flake.lock