Remove a possible existing store path when building CA derivations

In case a previous interrupted build left a garbage path laying around,
remove it before trying to move the path to its final location.

Fix #4858

tests/ca/build-with-garbage-path.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+
+# Regression test for https://github.com/NixOS/nix/issues/4858
+
+source common.sh
+sed -i 's/experimental-features .*/& ca-derivations ca-references/' "$NIX_CONF_DIR"/nix.conf
+
+# Get the output path of `rootCA`, and put some garbage instead
+outPath="$(nix-build ./content-addressed.nix -A rootCA --no-out-link)"
+nix-store --delete "$outPath"
+touch "$outPath"
+
+# The build should correctly remove the garbage and put the expected path instead
+nix-build ./content-addressed.nix -A rootCA --no-out-link
+
+# Rebuild it. This shouldn’t overwrite the existing path
+oldInode=$(stat -c '%i' "$outPath")
+nix-build ./content-addressed.nix -A rootCA --no-out-link --arg seed 2
+newInode=$(stat -c '%i' "$outPath")
+[[ "$oldInode" == "$newInode" ]]