LD_LIBRARY_PATH shouldn't be applied to touch command

[Kan Li <likan_999.student@…>]

What causes problem is this line: ​https://github.com/boostorg/build/blob/09b6788/src/tools/gcc.jam#L228

The line adds the lib directories of custom toolchain to LD_LIBRARY_PATH before running: some-binary-built-by-boost.build && touch some-file.passed

For my case, it causes problem because my custom toolchain comes with a newer version of glibc. It has no problem running the binary under the modified LD_LIBRARY_PATH, but touch command complains about the new glibc.

Logically speaking, only the first part requires the new LD_LIBRARY_PATH, while the touch command is pre-installed so it shouldn't be run under the modified LD_LIBRARY_PATH.