fix unix stdlib install location after #21328 (#24460)

closes #22369, closes #23197, closes #24385, refs #21328

According to #21328 the standard library on Unix should be installed in
`/usr/lib/nim/lib`, however the installer was not updated for this
change, hence the problem as described in
#23197 (comment).

Have not tested if this fixes the problem but the comment heavily
implies it does. The problem is also in 2.0 so it could be backported
but I can't say for sure that it works and doesn't break anything.

(cherry picked from commit 33e455c)

tools/niminst/install.nimf

@@ -31,15 +31,15 @@ if [ $# -eq 1 ] ; then
     "/usr/bin")
       bindir=$1
       configdir="/etc/?proj"
-      libdir="/usr/lib/?proj"
+      libdir="/usr/lib/?proj/lib"
       docdir="/usr/share/?proj/doc"
       datadir="/usr/share/?proj/data"
       nimbleDir="/opt/nimble/pkgs/?c.nimblePkgName-?c.version"
       ;;
     "/usr/local/bin")
       bindir=$1
       configdir="/etc/?proj"
-      libdir="/usr/local/lib/?proj"
+      libdir="/usr/local/lib/?proj/lib"
       docdir="/usr/local/share/?proj/doc"
       datadir="/usr/local/share/?proj/data"
       nimbleDir="/opt/nimble/pkgs/?c.nimblePkgName-?c.version"