tools/ci-build: install to a different directory for each build

This commit ensures the install directory will be distinct for each build,
with the two following motivations:

1. If required, it will make it possible to have several builds running
concurrently on the same slave without conflicts between their make install

2. So far, if for any reason make install fails, then the build script
gets stopped and the install directory will not be removed.
Thus if a future build tries to install to the same directory, its
installation could in principle fail, not because it is broken but because
the previous build has left the install directory in a corrupted state.

So this commit ensures that make install will be done in an empty
directory.

tools/ci-build

@@ -131,6 +131,9 @@ case "${OCAML_ARCH}" in
   *) arch_error;;
 esac
 
+# Make sure two builds won't use the same install directory
+instdir="$instdir-$$"
+
 #########################################################################
 # On Windows, cleanup processes that may remain from previous run