fix: Disable caching for modified source/include files

fix: Disable caching for modified source/include files

On a cache miss, ccache opts out of the direct mode when a "too new"
include file is found. This is done to avoid a race condition in the
direct mode when a header file if modified after ccache is invoked but
before the compiler is executed. Modifying the main source code file is
not a race condition when "run_second_cpp = false", which was the case
at the time the direct mode was introduced. However, the default of
run_second_cpp was changed to true in ccache 3.3, and the potiential
race condition then exists for both the source and the include files.

Fix this by disabling caching completely (not only the direct mode) when
modification of a source/include file is detected.

Closes #1378.