enhance download_file to fallback to https://ftp.gnu.org if downloading from https://ftpmirror.gnu.org fails

[boegel]

No description provided.

[smoors]

lgtm

[smoors]

tested with eb GCCcore-14.2.0.eb --fetch --force-download --rebuild. works well, but there are a few improvements that we can make:

• update the easyconfig to avoid trying source_urls for sources that we know aren't there.
• add trace output for the failed ftpmirror downloads

WARNING: Found file gcc-14.2.0.tar.gz at g/GCCcore/gcc-14.2.0.tar.gz, but re-downloading it anyway...

  >> download succeeded: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/gcc-14.2.0.tar.gz

WARNING: Found file gmp-6.3.0.tar.bz2 at g/GCCcore/gmp-6.3.0.tar.bz2, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/gmp-6.3.0.tar.bz2
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/gmp-6.3.0.tar.bz2
  >> download succeeded: https://ftp.gnu.org/gnu/gmp/gmp-6.3.0.tar.bz2

WARNING: Found file mpfr-4.2.1.tar.bz2 at g/GCCcore/mpfr-4.2.1.tar.bz2, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/mpfr-4.2.1.tar.bz2
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/mpfr-4.2.1.tar.bz2
  >> download failed: https://ftp.gnu.org/gnu/gmp/mpfr-4.2.1.tar.bz2
  >> download succeeded: https://ftp.gnu.org/gnu/mpfr/mpfr-4.2.1.tar.bz2

WARNING: Found file mpc-1.3.1.tar.gz at g/GCCcore/mpc-1.3.1.tar.gz, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/mpc-1.3.1.tar.gz
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/mpc-1.3.1.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/gmp/mpc-1.3.1.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/mpfr/mpc-1.3.1.tar.gz
  >> download succeeded: https://ftp.gnu.org/gnu/mpc/mpc-1.3.1.tar.gz

WARNING: Found file isl-0.26.tar.bz2 at g/GCCcore/isl-0.26.tar.bz2, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/isl-0.26.tar.bz2
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/isl-0.26.tar.bz2
  >> download failed: https://ftp.gnu.org/gnu/gmp/isl-0.26.tar.bz2
  >> download failed: https://ftp.gnu.org/gnu/mpfr/isl-0.26.tar.bz2
  >> download failed: https://ftp.gnu.org/gnu/mpc/isl-0.26.tar.bz2
  >> download failed: ftp://gcc.gnu.org/pub/gcc/infrastructure/isl-0.26.tar.bz2
  >> download failed: https://gcc.gnu.org/pub/gcc/infrastructure/isl-0.26.tar.bz2
  >> download succeeded: https://libisl.sourceforge.io/isl-0.26.tar.bz2

WARNING: Found file newlib-4.4.0.20231231.tar.gz at g/GCCcore/newlib-4.4.0.20231231.tar.gz, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/gmp/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/mpfr/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/mpc/newlib-4.4.0.20231231.tar.gz
  >> download failed: ftp://gcc.gnu.org/pub/gcc/infrastructure/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://gcc.gnu.org/pub/gcc/infrastructure/newlib-4.4.0.20231231.tar.gz
  >> download failed: https://libisl.sourceforge.io/newlib-4.4.0.20231231.tar.gz
  >> download succeeded: https://sourceware.org/pub/newlib/newlib-4.4.0.20231231.tar.gz

WARNING: Found file nvptx-tools-20240801.tar.gz at g/GCCcore/nvptx-tools-20240801.tar.gz, but re-downloading it anyway...

  >> download failed: https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/3136cf9.tar.gz
  >> download failed: https://sourceware.org/pub/gcc/releases/gcc-14.2.0/3136cf9.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/gmp/3136cf9.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/mpfr/3136cf9.tar.gz
  >> download failed: https://ftp.gnu.org/gnu/mpc/3136cf9.tar.gz
  >> download failed: ftp://gcc.gnu.org/pub/gcc/infrastructure/3136cf9.tar.gz
  >> download failed: https://gcc.gnu.org/pub/gcc/infrastructure/3136cf9.tar.gz
  >> download failed: https://libisl.sourceforge.io/3136cf9.tar.gz
  >> download failed: https://sourceware.org/pub/newlib/3136cf9.tar.gz
  >> download succeeded: https://github.com/MentorEmbedded/nvptx-tools/archive/3136cf9.tar.gz