Remove unnecessary executable flags from Rust source files (#12707)

# Objective
I found that some .rs files are unnecessarily executable.

Rust source files may start with a shebang-like statement `#!`, so let's
make sure they are not executable just in case.

Here is the result of the `find` commend that lists executable .rs files
as of main branch `86bd648`.
```console
$ find -name \*.rs -type f -executable
./crates/bevy_gizmos/src/lib.rs
./crates/bevy_tasks/src/lib.rs
./crates/bevy_time/src/lib.rs
./crates/bevy_transform/src/lib.rs
./src/lib.rs
```

It appears that the permissions of those files were originally 644, but
were unexpectedly changed to 755 by commit
52e3f20.

## Solution
Make them not executable by using this command;
`find -name \*.rs -type f -executable -exec chmod --verbose a-x -- {}
\+`