Make it possible to pass enabled injections at process startup via env
variables.

This patch fixes a couple of warnings that look like this:

```
warning: creating a shared reference to mutable static is discouraged
  --> src/error_injection.rs:23:43
   |
23 |     let Some(injected_errors) = (unsafe { &INJECTED_ERRORS }) else {
   |                                           ^^^^^^^^^^^^^^^^ shared reference to mutable static
   |
   = note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
   = note: this will be a hard error in the 2024 edition
   = note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
   |
23 |     let Some(injected_errors) = (unsafe { addr_of!(INJECTED_ERRORS) }) else {
   |                                           ~~~~~~~~~~~~~~~~~~~~~~~~~
```