Remove unused nightly toolchain environmental variable (bevyengine#13037

)

# Objective

- Many of our CI workflows contain a `NIGHTLY_TOOLCHAIN` environmental
variable.
- This specifies which nightly to use. If there is a bug in one of the
nightlies, we can pin the toolchain to be an earlier version.
- Both the daily and weekly workflows do not use the nightly compiler,
but still have a `NIGHTLY_TOOLCHAIN` variable.

## Solution

- Delete the unused variable.

.github/workflows/daily.yml

@@ -7,7 +7,6 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
-  NIGHTLY_TOOLCHAIN: nightly
 
 jobs:
   build-for-iOS:

.github/workflows/weekly.yml

@@ -9,7 +9,6 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
-  NIGHTLY_TOOLCHAIN: nightly
 
 jobs:
   test: