ci: shard linux gnu nextest tests into 2 partitions

.github/scripts/enable-unprivileged-userns.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Required for bubblewrap to work on Linux CI runners.
+sudo sysctl -w kernel.unprivileged_userns_clone=1
+
+# Ubuntu 24.04+ can additionally gate unprivileged user namespaces behind AppArmor.
+if sudo sysctl -a 2>/dev/null | grep -q '^kernel.apparmor_restrict_unprivileged_userns'; then
+  sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0
+fi

.github/scripts/nextest-archive-build.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+archive_file="$1"
+target="${2:-x86_64-unknown-linux-gnu}"
+cargo_profile="${3:-ci-test}"
+
+cargo nextest archive \
+  --all-features \
+  --target "$target" \
+  --cargo-profile "$cargo_profile" \
+  --timings \
+  --archive-file "$archive_file"

.github/scripts/nextest-archive-run-shard.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+archive_file="$1"
+workspace_remap="$2"
+partition_spec="$3"
+
+cargo nextest run \
+  --archive-file "$archive_file" \
+  --workspace-remap "$workspace_remap" \
+  --partition "$partition_spec" \
+  --no-fail-fast