Disable clang-tidy by default

Summary: While this doesn't slow down a fresh build by much, this is disastrous when jumping between branches as none of the results are cached and they all need to be recomputed.

Test Plan:
Run ninja clean and then ninja with clang-tidy:

real    1m22.667s
user    11m38.143s
sys     0m37.996s

Now the same without clang-tidy:

real    0m28.937s
user    0m46.723s
sys     0m10.769s

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5163