run-tests: fix 'eval' command for newer versions of nix

There's no common `nix` command argument syntax for eval'ing a nix
expression that supports both older and newer (flake support) versions of nix.
So fall back to nix-instantiate.

test/run-tests.sh

@@ -165,8 +165,7 @@ debug() {
 }
 
 evalTest() {
-    nix eval --raw "($(vmTestNixExpr)).outPath"
-    echo # nix eval doesn't print a newline
+    nix-instantiate --eval -E "($(vmTestNixExpr)).outPath"
 }
 
 instantiate() {