bump pinned nightly from nightly-2020-12-09 to nightly-2020-12-14

This should hopefully fix incremental compilation ICEs from rustc that I have been encountering multiple times while working with the previous nightly.

Author: matthiaskrgr

rust-toolchain

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2020-12-09"
+channel = "nightly-2020-12-14"
 components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]