Support SLANG_PLUGIN_PATH env var for custom slang plugin location

When SYNTH_HDL_FRONTEND=slang, the plugin load is hardcoded as
`plugin -i slang`.  This adds a check for SLANG_PLUGIN_PATH,
falling back to the current behavior when unset.

This enables Bazel-built slang plugins to be loaded from a
custom path, which is needed for downstream consumers that
build yosys-slang from source.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/scripts/synth_preamble.tcl

@@ -44,7 +44,11 @@ proc read_design_sources { } {
   }
 
   if { [env_var_equals SYNTH_HDL_FRONTEND slang] } {
-    plugin -i slang
+    if { [info exists ::env(SLANG_PLUGIN_PATH)] } {
+      plugin -i $::env(SLANG_PLUGIN_PATH)
+    } else {
+      plugin -i slang
+    }
 
     set slang_args [list \
       -D SYNTHESIS --keep-hierarchy --compat=vcs --ignore-assertions --top $::env(DESIGN_NAME) \