Fix building CA derivations with and eval store

I don't love the way this code looks. There are two larger problems:

- eval, build/scratch, destination stores (#5025) should have different
  types to reflect the fact that they are used for different purposes
  and those purposes correspond to different operations. It should be
  impossible to "use the wrong store" in my cases.

- Since drvs can end up in both the eval and build/scratch store, we
  should have some sort of union/layered store (not on the file sytem
  level, just conceptual level) that allows accessing both. This would
  get rid of the ugly "check both" boilerplate in this PR.

Still, it might be better to land this now / soon after minimal cleanup,
so we have a concrete idea of what problem better abstractions are
supposed to solve.

tests/functional/ca/eval-store.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+# Ensure that garbage collection works properly with ca derivations
+
+source common.sh
+
+export NIX_TESTS_CA_BY_DEFAULT=1
+
+cd ..
+source eval-store.sh

tests/functional/ca/local.mk

@@ -5,6 +5,7 @@ ca-tests := \
   $(d)/concurrent-builds.sh \
   $(d)/derivation-json.sh \
   $(d)/duplicate-realisation-in-closure.sh \
+  $(d)/eval-store.sh \
   $(d)/gc.sh \
   $(d)/import-derivation.sh \
   $(d)/new-build-cmd.sh \

tests/functional/eval-store.sh

@@ -11,7 +11,16 @@ rm -rf "$eval_store"
 
 nix build -f dependencies.nix --eval-store "$eval_store" -o "$TEST_ROOT/result"
 [[ -e $TEST_ROOT/result/foobar ]]
-(! ls $NIX_STORE_DIR/*.drv)
+if [[ ! -n "${NIX_TESTS_CA_BY_DEFAULT:-}" ]]; then
+    # Resolved CA derivations are written to store for building
+    #
+    # TODO when we something more systematic
+    # (https://github.com/NixOS/nix/issues/5025) that distinguishes
+    # between scratch storage for building and the final destination
+    # store, we'll be able to make this unconditional again -- resolved
+    # derivations should only appear in the scratch store.
+    (! ls $NIX_STORE_DIR/*.drv)
+fi
 ls $eval_store/nix/store/*.drv
 
 clearStore
@@ -26,5 +35,8 @@ rm -rf "$eval_store"
 
 nix-build dependencies.nix --eval-store "$eval_store" -o "$TEST_ROOT/result"
 [[ -e $TEST_ROOT/result/foobar ]]
-(! ls $NIX_STORE_DIR/*.drv)
+if [[ ! -n "${NIX_TESTS_CA_BY_DEFAULT:-}" ]]; then
+    # See above
+    (! ls $NIX_STORE_DIR/*.drv)
+fi
 ls $eval_store/nix/store/*.drv