fix: flakes now obey --eval-system

This required changing an excessive number of places since flakes code
is a delicious bowl of copy pasta.

I didn't change all of the usage sites since some of them run things on
the local machine and you actually want it to be the real system there.

Nevertheless, we probably should have the daemon do something much
different when it receives a `system` setting:
https://git.lix.systems/lix-project/lix/issues/694

Fixes: https://git.lix.systems/lix-project/lix/issues/692
Fixes: https://git.lix.systems/lix-project/lix/issues/673
Fixes: https://github.com/NixOS/nix/issues/11359
Change-Id: I55e696c09794d2520b60238a84829c98fcad7ccc

tests/functional/flakes/check.sh

@@ -107,6 +107,11 @@ cat > $flakeDir/flake.nix <<EOF
 EOF
 
 nix flake check $flakeDir
+# --eval-system should be considered for which the local system is for flake
+# purposes, and thus it should fail checking that attr
+(! nix flake check --eval-system system-1 $flakeDir)
+# likewise with --system
+(! nix flake check --system system-1 $flakeDir)
 
 checkRes=$(nix flake check --all-systems --keep-going $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
 echo "$checkRes" | grepQuiet "packages.system-1.default"

tests/functional/flakes/flakes.sh

@@ -126,6 +126,11 @@ nix build -o $TEST_ROOT/result git+file://$flake1Dir
 nix build -o $flake1Dir/result git+file://$flake1Dir
 nix path-info $flake1Dir/result
 
+# Ensure that eval-system affects the chosen attribute
+cp -r "$flake1Dir" "$flake1Dir.kittified"
+sed -i "s#$system#kitty-kitty#" "$flake1Dir.kittified/flake.nix"
+nix build --eval-system kitty-kitty "$flake1Dir.kittified"
+
 # 'getFlake' on an unlocked flakeref should fail in pure mode, but
 # succeed in impure mode.
 (! nix build -o $TEST_ROOT/result --expr "(builtins.getFlake \"$flake1Dir\").packages.$system.default")

tests/functional/flakes/show.sh

@@ -7,6 +7,8 @@ writeSimpleFlake "$flakeDir"
 cd "$flakeDir"
 
 
+# FIXME(jade): the following is rather absurd. we have jq!
+
 # By default: Only show the packages content for the current system and no
 # legacyPackages at all
 nix flake show --json > show-output.json
@@ -19,6 +21,11 @@ assert show_output.legacyPackages.${builtins.currentSystem} == {};
 true
 '
 
+# Follow --eval-system for determining the system for flakes
+nix flake show --eval-system someOtherSystem --json > show-output.json
+drvTitle=$(jq -r '.packages.someOtherSystem.default.name' show-output.json)
+[[ $drvTitle == 'simple' ]]
+
 # With `--all-systems`, show the packages for all systems
 nix flake show --json --all-systems > show-output.json
 nix eval --impure --expr '