Re #7469: doc: unified references to PATH environment variable

Found four variants, picked the first:

  1. ``PATH``
  2. `PATH`
  3. ``$PATH``
  4. :envvar:`PATH`

The last (4) produced an error with sphinx:
```
doc/cabal-project.rst:644: WARNING: 'envvar' reference target not found: PATH
```

doc/cabal-package.rst

@@ -1821,12 +1821,12 @@ system-dependent values for these fields.
         build-tool-depends: mypackage:executable
 
     Cabal tries to make sure that all specified programs are atomically built and prepended
-    on the ``$PATH`` shell variable before building the component in question, but can only do
+    on the ``PATH`` shell variable before building the component in question, but can only do
     so for Nix-style builds. Specifically:
 
     a) For Nix-style local builds, both internal and external dependencies.
     b) For old-style builds, only for internal dependencies [#old-style-build-tool-depends]_.
-       It's up to the user to provide needed executables in this case under `$PATH.`
+       It's up to the user to provide needed executables in this case under ``PATH``.
 
 
     .. note::
@@ -3489,5 +3489,5 @@ For more information about Backpack, check out the
   Some packages (ab)use :pkg-field:`build-depends` on old-style builds, but this has a few major drawbacks:
 
     - using Nix-style builds it's considered an error if you depend on a exe-only package via build-depends: the solver will refuse it.
-    - it may or may not place the executable on ``$PATH``.
+    - it may or may not place the executable on ``PATH``.
     - it does not ensure the correct version of the package is installed, so you might end up overwriting versions with each other.

doc/cabal-project.rst

@@ -642,7 +642,7 @@ feature was added.
     :synopsis: Path to compiler executable.
 
     Specify the path to a particular compiler to be used. If not an
-    absolute path, it will be resolved according to the :envvar:`PATH`
+    absolute path, it will be resolved according to the ``PATH``
     environment. The type of the compiler (GHC, GHCJS, etc) must be
     consistent with the setting of the :cfg-field:`compiler` field.
 

doc/developing-packages.rst

@@ -230,7 +230,7 @@ If the package contains an executable, it can be installed:
     $ cabal install
 
 The executable program lands in a special directory for binaries
-that may or may not already be on your system's ``$PATH``.
+that may or may not already be on your system's ``PATH``.
 If it is, the executable can be run by typing its filename on commandline.
 For installing libraries see the :ref:`adding-libraries` section.