setup-hook.sh: use eval for dynamic variable export

pkgs/build-support/bintools-wrapper/setup-hook.sh

@@ -1,3 +1,4 @@
+# shellcheck shell=bash
 # Binutils Wrapper hygiene
 #
 # See comments in cc-wrapper's setup hook. This works exactly the same way.
@@ -14,7 +15,9 @@ bintoolsWrapper_addLDVars () {
     getHostRoleEnvHook
 
     if [[ -d "$1/lib64" && ! -L "$1/lib64" ]]; then
-        export NIX_LDFLAGS${role_post}+=" -L$1/lib64"
+        varName=NIX_LDFLAGS${role_post}
+        eval "$varName=\"${!varName:-} -L$1/lib64\""
+        export "${varName?}"
     fi
 
     if [[ -d "$1/lib" ]]; then
@@ -24,7 +27,9 @@ bintoolsWrapper_addLDVars () {
         # directories and bloats the size of the environment variable space.
         local -a glob=( $1/lib/lib* )
         if [ "${#glob[*]}" -gt 0 ]; then
-            export NIX_LDFLAGS${role_post}+=" -L$1/lib"
+            varName=NIX_LDFLAGS${role_post}
+            eval "$varName=\"${!varName:-} -L$1/lib\""
+            export "${varName?}"
         fi
     fi
 }