BF: install_yn is currently 'lost' between shell commands

The current tclinstall rule looks to be failing due to install_yn not being propagated between shell commands. This breaks the test that runs auto_mkindex, example output from a make install below

Installing tcl scripts
install -p -m 0755 feat.tcl <blah>/tcl/
...
ln -s Runtcl Feat_gui
...
/bin/sh: line 0: [: =: unary operator expected

assigned to @paulmc

I must have accidentally introduced this when cleaning up the rules file - original code here: https://git.fmrib.ox.ac.uk/fsl/config/-/blob/master/common/rules.mk#L184-194

merged