Trac #32784: filter singular in tox.ini for homebrew.

The homebrew singular package doesn't support using a prefix other
than /usr/local, and so cannot be used for our existing GH actions.
Here we filter it out of the list of $PACKAGES to be installed.

tox.ini

@@ -510,7 +510,9 @@ commands =
     #
     # https://docs.brew.sh/Installation
     homebrew:      bash -c 'if [ ! -x {env:HOMEBREW}/bin/brew ]; then mkdir -p {env:HOMEBREW} && cd {env:HOMEBREW} && curl -L https://github.com/Homebrew/brew/tarball/master | tar xz --strip 1 ; fi'
-    homebrew:      bash -c 'case "{env:SKIP_SYSTEM_PKG_INSTALL:}" in 1|y*|Y*);; *) PACKAGES=$(build/bin/sage-get-system-packages homebrew $(PATH=build/bin:$PATH build/bin/sage-package list {env:SAGE_PACKAGE_LIST_ARGS}) {env:EXTRA_SAGE_PACKAGES}); {env:HOMEBREW}/bin/brew install $PACKAGES; {env:HOMEBREW}/bin/brew upgrade $PACKAGES;; esac'
+    # Homebrew's singular doesn't support using a prefix other than
+    # /usr/local, so we filter it out of the list here.
+    homebrew:      bash -c 'case "{env:SKIP_SYSTEM_PKG_INSTALL:}" in 1|y*|Y*);; *) PACKAGES=$(build/bin/sage-get-system-packages homebrew $(PATH=build/bin:$PATH build/bin/sage-package list {env:SAGE_PACKAGE_LIST_ARGS}) {env:EXTRA_SAGE_PACKAGES}); PACKAGES=${PACKAGES/singular /}; {env:HOMEBREW}/bin/brew install $PACKAGES; {env:HOMEBREW}/bin/brew upgrade $PACKAGES;; esac'
     #
     # local-conda
     #