Fails to find a static library with runtime-link=shared

B2 fails to find static library (zlib) when I change `runtime-link=static` to `runtime-link=shared`:

```
$ ls /path/to/include/x64-debug
zconf.h zlib.h

$ ls /path/to/lib/x64-debug
libz.a

$ ./b2 -n --reconfigure --debug-configure --with-iostreams -s ZLIB_INCLUDE=/path/to/include/x64-debug -s ZLIB_LIBPATH=/path/to/lib/x64-debug -s ZLIB_NAME=libz link=static runtime-link=static

notice: [zlib] Using pre-installed library
notice: [zlib] Condition

Building the Boost C++ Libraries.
- zlib : yes [3]

$ ./b2 -n --reconfigure --debug-configure --with-iostreams -s ZLIB_INCLUDE=/path/to/include/x64-debug -s ZLIB_LIBPATH=/path/to/lib/x64-debug -s ZLIB_NAME=libz link=static runtime-link=shared

notice: [zlib] Using pre-installed library
notice: [zlib] Condition

Building the Boost C++ Libraries.
- zlib : no [3]

[3] link=static
```

No other difference in command line nor in environment, other than `runtime-link=static` vs. `runtime-link=shared`.

Tried current tip of the master branch (2b06d9f20560a4f7292833fcb71d8c6a3652ca76) as well as an old version 1.71.0.

I expect C++ runtime type selection to not affect library search.