Update the CI to use the latest Nim nightly build

The Nimble CI is updated to use the currently latest Nim nightly build
from 18th of July 2021.

Related to #127

Author: bobeff

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
           - macos-latest
           - ubuntu-latest
         nimversion:
-          - sourcetar:https://github.com/bobeff/Nim/tarball/8ede3bc0494aa27d98384fa5449c5c3026ac8f92
+          - nightly:https://github.com/nim-lang/nightlies/releases/tag/2021-07-18-devel-923a1c6ea7d9f45b6389680717692690218228fb
     name: ${{ matrix.os }} - ${{ matrix.nimversion }}
     runs-on: ${{ matrix.os }}
     env: