Changes to PATH quietly ignored

**Describe the bug**
`cabal v2-build` caches the `PATH` environment variable and does not always pass on changes (perhaps unless they impact program selection). Other environment variable changes are passed on. This is confusing for tools like `ghcjs-hsc2hs` that wrap other executables and issue errors when they are not in the `PATH`. Updating the `PATH` and rerunning `cabal v2-build` does not work without a `rm -rf dist-newstyle` because `cabal v2-build` continues to use the old `PATH`.

**To Reproduce**
Steps to reproduce the behavior:

Here is an example of the new `PATH` value being ignored (while `NOTPATH` is not). Adding a bug to the source makes sure `cabal` prints the environment instead of just skipping the step that does that.

```
~/haskell$ cabal unpack hello
Unpacking to hello-1.0.0.2/
~/haskell$ cd hello-1.0.0.2
~/haskell/hello-1.0.0.2$ echo bug >> src/hello.hs
~/haskell/hello-1.0.0.2$ cabal v2-build --verbose=3 2>&1 | grep HELLO
~/haskell/hello-1.0.0.2$ PATH=HELLO:$PATH cabal new-build --verbose=3 2>&1 | grep HELLO
~/haskell/hello-1.0.0.2$ NOTPATH=HELLO cabal new-build --verbose=3 2>&1 | grep HELLO
Environment: [[dump of the environment including HELLO]]
```

**Expected behavior**
I would have expected the `Environment:` to include `HELLO` in both cases, but it only does when the `NOTPATH` variable is set. Changes in `PATH` should be passed on to child processes even if they do not impact cabal's program selection.

**System information**
```
~/haskell/hello-1.0.0.2$ cabal --version
cabal-install version 3.0.0.0
compiled using version 3.0.0.0 of the Cabal library
~/haskell/hello-1.0.0.2$ uname -a
Darwin MacBook-Pro-4.local 18.7.0 Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64 x86_64
~/haskell/hello-1.0.0.2$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5
```