testsuite fails with ghc-9.12.2

Stackage Nightly say:

```
Test suite failure for package read-env-var-1.0.0.0
read-env-var-doctest: exited with: ExitFailure 1
Full log available at /home/curators/work/unpack-dir/.stack-work/logs/read-env-var-1.0.0.0-test.log


src/System/ReadEnvVar.hs:214: failure in expression `lookupEnvEx "THIS_ENV_VAR_WILL_NOT_EXIST" :: IO String'
expected: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"
but got: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"

^
HasCallStack backtrace:
throwIO, called at libraries/exceptions/src/Control/Monad/Catch.hs:308:12 in exceptions-0.10.9-687c:Control.Monad.Catch
throwM, called at src/System/ReadEnvVar.hs:223:16 in main:System.ReadEnvVar


src/System/ReadEnvVar.hs:240: failure in expression `readEnvEx "THIS_ENV_VAR_WILL_NOT_EXIST" :: IO (Maybe Int)'
expected: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"
but got: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"

^
HasCallStack backtrace:
throwIO, called at libraries/exceptions/src/Control/Monad/Catch.hs:308:12 in exceptions-0.10.9-687c:Control.Monad.Catch
throwM, called at src/System/ReadEnvVar.hs:268:16 in main:System.ReadEnvVar


src/System/ReadEnvVar.hs:296: failure in expression `readEnvEx' "THIS_ENV_VAR_WILL_NOT_EXIST" :: IO Int'
expected: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"
but got: *** Exception: EnvVarDoesNotExistException "THIS_ENV_VAR_WILL_NOT_EXIST"

^
HasCallStack backtrace:
throwIO, called at libraries/exceptions/src/Control/Monad/Catch.hs:308:12 in exceptions-0.10.9-687c:Control.Monad.Catch
throwM, called at src/System/ReadEnvVar.hs:268:16 in main:System.ReadEnvVar


Examples: 45 Tried: 33 Errors: 0 Failures: 3
```

Start with the doctest expressions at src/System/ReadEnvVar.hs lines 214, 240, and 296, then run the read-env-var-doctest suite under GHC 9.12.2. Compare the expected and actual EnvVarDoesNotExistException output and determine what change makes all three examples pass.

Written by the indexing model from the issue text.