run_target() does some variable substitutions since 0.57.0. This is a
new behavior, and undocumented, caused by sharing more code with
custom_target(). More consistency is better, so document it now.
custom_target() was doing variable substitution in the past, because it
shared some code with generator(), but that was undocumented. Some
refactoring in 0.57.0 caused it to not replace @CURRENT_SOURCE_DIR@,
@SOURCE_DIR@, and @BUILD_DIR@ anymore. This patch adds back
@CURRENT_SOURCE_DIR@ and document it. It does not add back @SOURCE_DIR@
because it is duplicate with @SOURCE_ROOT@ that has a better name. Also
do not add back @BUILD_DIR@ which is duplicate of @PRIVATE_DIR@, and
not @BUILD_ROOT@ surprisingly, adding to the confusion.
Re-implement it in backend using the same code path as for
custom_target(). This for example handle setting PATH on Windows when
command is an executable.