cacert: decouple from NSS to reduce rebuild amount

In [#100765] @vcunat pointed out that we could decouple cacert from the
NSS package to make it more rebuild friendly. Just rebuilding packages
that depend on NSS seems to be about ~100. Rebuilding all the packages
that depend on cacert is >9k as of this writing. This makes it much more
feasible to upgrade high-profile packages that are (rightfully) pedantic
on their NSS version like firefox and thunderbird.

[#100765]: https://github.com/NixOS/nixpkgs/pull/100765

pkgs/data/misc/cacert/update.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p nix common-updater-scripts jq
+
+# Build both the cacert package and an overriden version where we use the source attribute of NSS.
+# Cacert and NSS are both from the same upstream sources. They are decoupled as
+# the cacert output only cares about a few infrequently changing files in the
+# sources while the NSS source code changes frequently.
+#
+# By having cacert on a older source revision that produces the same
+# certificate output as a newer version we can avoid large amounts of
+# unnecessary rebuilds.
+#
+# As of this writing there are a few magnitudes more packages depending on
+# cacert than on nss.
+
+
+set -ex
+
+BASEDIR="$(dirname "$0")/../../../.."
+
+
+CURRENT_PATH=$(nix-build --no-out-link -A cacert.out)
+PATCHED_PATH=$(nix-build --no-out-link -E "with import $BASEDIR {}; (cacert.overrideAttrs (_: { inherit (nss) src version; })).out")
+
+# Check the hash of the etc subfolder
+# We can't check the entire output as that contains the nix-support folder
+# which contains the output path itself.
+CURRENT_HASH=$(nix-hash "$CURRENT_PATH/etc")
+PATCHED_HASH=$(nix-hash "$PATCHED_PATH/etc")
+
+if [[ "$CURRENT_HASH" !=  "$PATCHED_HASH" ]]; then
+    NSS_VERSION=$(nix-instantiate --json --eval -E "with import $BASEDIR {}; nss.version" | jq -r .)
+    update-source-version cacert "$NSS_VERSION"
+fi