[CI] Allow configuration script to override CI defaults.

[bader]

When we originally discussed --ci-defaults option, I thought that these defaults can be overridden by additional options. E.g. --ci-defaults --option-X=val, so that if --ci-defaults sets --option-X, users can replace the default value.
Unfortunately, I don't see this requirement to be documented in the code and it seems to be not true for options modified inside this if-statement.

Originally posted by @bader in #5091 (comment)

[uditagarwal97]

Hi @bader, I don't completely understand the motivation behind this request. AFAIK, we use --ci-defaults only in SYCLOS CI. If that is indeed the case, why/when/who would need the capability to override the options set by --ci-defaults?

[bader]

DPC++ developers who replicate CI behavior but need to tweak their cmake configuration.