CI: use different nightlies again

Our nightlies were all fixed at 2021-01-21 because we moved
the minimum `rustc` version to that one right after it was
released. Now that some weeks have passed, we can spread
a few different nightlies again.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

.github/workflows/ci.yaml

@@ -27,23 +27,23 @@ jobs:
           - arch: x86_64
             toolchain: gcc
             config: debug
-            rustc: 2021-01-24
+            rustc: 2021-01-31
             output: build
             install: rustup
             sysroot: custom
 
           - arch: arm64
             toolchain: clang
             config: release
-            rustc: 2021-01-24
+            rustc: 2021-02-07
             output: build
             install: standalone
             sysroot: common
 
           - arch: x86_64
             toolchain: llvm
             config: debug
-            rustc: 2021-01-24
+            rustc: 2021-02-11
             output: build
             install: standalone
             sysroot: custom