CI: Add "tree" to Cygwin build dependencies for better build diagnostics

Author: mesheets

.github/workflows/gnu-legacy-toolchain_CI.yml

@@ -30,7 +30,7 @@ env:
   # Cygwin-official Action: https://github.com/marketplace/actions/install-cygwin-action
   # ** NOTE **: On Cygwin, need to create a symlink for libtermcap.a -> libncurses.a
   #  - https://superuser.com/a/1020342
-  CYGWIN_BUILD_DEPENDS: git autoconf automake m4 make binutils gcc-core gcc-g++ gcc-objc bison flex libtool gawk gettext gettext-devel gettext-debuginfo texinfo  file libisl-devel libncurses-devel dejagnu  stow
+  CYGWIN_BUILD_DEPENDS: git autoconf automake m4 make binutils gcc-core gcc-g++ gcc-objc bison flex libtool gawk gettext gettext-devel gettext-debuginfo texinfo  file libisl-devel libncurses-devel dejagnu  stow tree
   CYGWIN_GCC44_BUILD_DEPENDS: libglib2.0-devel libgmp-devel libmpfr-devel libmpc-devel zip unzip
   # Cygwin lacks packages for Mono, so the "gcc44" toolchain set is unsupported unless Mono is built and installed manually
   CYGWIN_GCC44_MISSING_DEPENDS:    mono-runtime libmono-2.0-devel