Add a test for the “two glibc” issue

tests/ca/duplicate-realisation-in-closure.sh

@@ -0,0 +1,26 @@
+source ./common.sh
+
+sed -i 's/experimental-features .*/& ca-derivations ca-references/' "$NIX_CONF_DIR"/nix.conf
+
+export REMOTE_STORE_DIR="$TEST_ROOT/remote_store"
+export REMOTE_STORE="file://$REMOTE_STORE_DIR"
+
+rm -rf $REMOTE_STORE_DIR
+clearStore
+
+# Build dep1 and push that to the binary cache.
+# This entails building (and pushing) current-time.
+nix copy --to "$REMOTE_STORE" -f nondeterministic.nix dep1
+clearStore
+sleep 2 # To make sure that `$(date)` will be different
+# Build dep2.
+# As we’ve cleared the cache, we’ll have to rebuild current-time. And because
+# the current time isn’t the same as before, this will yield a new (different)
+# realisation
+nix build -f nondeterministic.nix dep2
+
+# Build something that depends both on dep1 and dep2.
+# If everything goes right, we should rebuild dep2 rather than fetch it from
+# the cache (because that would mean duplicating `current-time` in the closure),
+# and have `dep1 == dep2`.
+nix build --substituters "$REMOTE_STORE" -f nondeterministic.nix toplevel --no-require-sigs