[CI] Do not rewrite links in symlinked files

[giordano]

If we rewrite the hyperlinks in symlinked files we risk to accidentally append duplicate ${PREVIEW_SUBDIR}, depending on the order with which find operates on the files.

Should fix #304.

[giordano]

@tkoskela I'm not crazy, right? This line after rewriting the links is

- [Installation](https://ukri-excalibur.github.io/excalibur-tests/preview/PR305/install/)

right? But then the generated website has the duplicate subdir

excalibur-tests/preview/PR305/index.html

Line 1212 in 5d49476

<li><a href="https://ukri-excalibur.github.io/excalibur-tests/preview/PR305/preview/PR305/install/">Installation</a></li>

[giordano]

As far as I understand: the link rewriting command is working as intended, but mkdocs sometimes is doing something stupid when generating the HTML files. I say "sometimes" because in #295 it was working correctly.

[giordano]

After a long time staring at the screen to spot the error, I believe now I understand the problem: the find command is editing all *.md files, but some of them are symlinks, and so there's the risk to rewrite the links twice if the original file is modified first, and the symlink is modified afterwards (once the symlink is modified by sed it's turned into a real file and the symlink is broken, if the order is inverted we wouldn't get double rewrite of the same file).

This probably also explains why the error was occurring occasionally: the occurrence of the error depends on the order with which find operates on the files, which is probably unspecified and can change from run to run.

Note that the links are finally correct in 22d3f8e (triggered by adf2e95) and stayed the same in the three subsequent commits (22d3f8e...3e11c4b)