Allow TeXLive mirror to be selected

[garethstockwell]

If set, the TEXLIVE_MIRROR_URL environment variable identifies the TeXLive mirror which should be used.

This is particularly useful when installing a historic TeXLive version. In this case, there is no automated mirror redirect, and the default mirror (ftp://tug.org/historic/) can become overloaded.

A mirror can be selected from https://www.tug.org/historic/, for example:

make latex
PANDOC_COMMIT=3.1.1
TEXLIVE_MIRROR_URL=https://ftp.math.utah.edu/pub/tex/historic/

When installing the most recent TeXLive version, by default the mirror with the fastest response is used, by resolving the mirror URL http://mirror.ctan.org/.

Alternatively, a mirror can be manually selected from https://ctan.org/mirrors/, for example:

make latex
TEXLIVE_MIRROR_URL=http://tug.ctan.org/tex-archive/