Specify virtualenv as extra dependency for build in tox.ini

`build` seems to rely by default in the stdlib's implementation of `venv`
and unfortunately for a specific combination of Windows + conda env + virtualenv (as used by tox),
that might be problematic as identified in pyscaffold/pyscaffoldext-markdown#10.

According to the discussion in pypa/build#253,
specifying `virtualenv` as an extra dependency of `build` seems to be a
good workaround. Moreover I would expect some kind speed improvements
(virtualenv is reported to be faster than venv).

src/pyscaffold/templates/tox_ini.template

@@ -30,7 +30,7 @@ description =
 skip_install = True
 changedir = {toxinidir}
 deps =
-    build: build
+    build: build[virtualenv]
 commands =
     clean: python -c 'from shutil import rmtree; rmtree("build", True); rmtree("dist", True)'
     build: python -m build .

tox.ini

@@ -52,7 +52,7 @@ description =
 skip_install = True
 changedir = {toxinidir}
 deps =
-    build: build
+    build: build[virtualenv]
 commands =
     clean: python -c 'from shutil import rmtree; rmtree("build", True); rmtree("dist", True)'
     build: python -m build .