build.plat: remove TemplatedPlatform.unix_interpreter.

Vendor toolchains generally require far more workarounds than this,
and we already have a perfectly fine way of overriding templates.

nmigen/vendor/xilinx_spartan_3_6.py

@@ -76,6 +76,13 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
 
     file_templates = {
         **TemplatedPlatform.build_script_templates,
+        "build_{{name}}.sh": r"""
+            # {{autogenerated}}
+            set -e{{verbose("x")}}
+            if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
+            [ -n "$NMIGEN_{{platform.toolchain}}_env" ] && . "$NMIGEN_{{platform.toolchain}}_env"
+            {{emit_commands("sh")}}
+        """,
         "{{name}}.v": r"""
             /* {{autogenerated}} */
             {{emit_design("verilog")}}
@@ -118,7 +125,6 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
             {{get_override("add_constraints")|default("# (add_constraints placeholder)")}}
         """
     }
-    unix_interpreter  = "bash"
     command_templates = [
         r"""
         {{get_tool("xst")}}