Prevent buggy interaction between MinGW and WSL

This changes the hashbangs in Makefile helper scripts to be static.

Often, "#!/usr/bin/env bash" is a better hashbang for bash scripts
than "#!/bin/bash", because it automatically works on Unix-like
systems that are not GNU/Linux and do not have bash in /bin, but
on which it has been installed in another $PATH directory, such as
/usr/local/bin. (It can also be helpful on macOS, where /bin/bash
is usually an old version of bash, while a package manager such as
brew may have been used to install a newer version elsewhere.)

Windows systems with WSL installed often have a deprecated bash.exe
in the System32 directory that runs commands and scripts inside an
installed WSL system. (wsl.exe should be used instead.) Anytime
that bash is used due to a "#!/usr/bin/env bash" hashbang, it is
wrong, because that only happens if the caller is some Unix-style
script running natively or otherwise outside WSL.

Normally this is not a reason to prefer a "#!/bin/bash" hashbang,
because normally any environment in which one can run a script in a
way that determines its interpreter from its hashbang is an
environment in which a native (or otherwise appropriate) bash
precedes the System32 bash in a PATH search. However, MinGW make,
a popular make implementation used on Windows, is an exception.

The goal of this change is not to sacrifice support for some
Unix-like systems to better support Windows, which wouldn't
necessarily be justified. Rather, this is to avoid extremely
confusing wrong behavior that in some cases would have bizarre
effects that are very hard to detect. I discovered this problem
because the VIRTUAL_ENV variable was not inheried by the bash
interpreter (because it was, fortunately, not passed through to
WSL). But if "python3 -m build" finds a global "build" package,
things might get much further before any problem is noticed.

build-release.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 #
 # This script builds a release. If run in a venv, it auto-installs its tools.
 # You may want to run "make release" instead of running this script directly.

check-version.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 #
 # This script checks if we are in a consistent state to build a new release.
 # See the release instructions in README.md for the steps to make this pass.