[build] fix nimbus-build-system (#343)

Ensures that we include the correct paths when building
with nimbus-build-system.

Fixes Error: cannot open file: ""

Copied from:
1ea102b033/config.nims (L5)

config.nims

@@ -1,5 +1,15 @@
 import std/os
 
+const currentDir = currentSourcePath()[0 .. ^(len("config.nims") + 1)]
+
+if getEnv("NIMBUS_BUILD_SYSTEM") == "yes" and
+   # BEWARE
+   # In Nim 1.6, config files are evaluated with a working directory
+   # matching where the Nim command was invocated. This means that we
+   # must do all file existance checks with full absolute paths:
+   system.fileExists(currentDir & "nimbus-build-system.paths"):
+  include "nimbus-build-system.paths"
+
 if defined(release):
   switch("nimcache", joinPath(currentSourcePath.parentDir, "nimcache/release/$projectName"))
 else: