chore(ci): raise the compiler version to nightly-2020-04-23

yvt · yvt · commit 12d2d23722bc · 2020-05-04T17:38:33.000+09:00
The latest available version is nightly-2020-04-30, but we can't use this because of <rust-lang/rust#71612>.
diff --git a/ci/common.yml b/ci/common.yml
@@ -1,5 +1,5 @@
 variables:
-  rustVersion: nightly-2020-02-05
+  rustVersion: nightly-2020-04-23
   linuxPrestep: |
     sudo apt-get update &&
     sudo apt-get install -y \
diff --git a/rust-toolchain b/rust-toolchain
@@ -1 +1 @@
-nightly-2020-02-05
+nightly-2020-04-23
