[icu] Fix path to fallback install-sh

[Kai Pastor (dg0yt)]

icu uses AC_PROG_INSTALL which tries to find a fast binary install and uses a slow install-sh script as fallback.
When the fallback is used, icu prepends $(top_srcdir) for (some) in-source builds. But vcpkg builds out-of-source, and the value INSTALL is valid only for the toplevel build dir.
So this patch prepends $(top_builddir) instead, so that a valid path is construct for every possible case.

To verify, rename /usr/bin/install or similar before installing icu. (E.g. apply

vcpkg_replace_string("${SOURCE_PATH}/source/configure" "DUALCASE=1" "mv /usr/bin/install /usr/bin/install.orig\nDUALCASE=1" IGNORE_UNCHANGED)

after vcpkg_extract_source_archive.)

Alternative to / closes #52618.
Fixes #51759.
Fixes #50967.
Fixes #48887.
Fixes #48141.
Fixes #46567.