diff --git a/nmigen/build/plat.py b/nmigen/build/plat.py
index c8e5058..1ad3999 100644
--- a/nmigen/build/plat.py
+++ b/nmigen/build/plat.py
@@ -60,12 +60,17 @@ class Platform(ResourceManager, metaclass=ABCMeta):
             raise TypeError("File contents must be str, bytes, or a file-like object")
         self.extra_files[filename] = content
 
+    @property
+    def _toolchain_env_var(self):
+        return f"NMIGEN_{self.toolchain}_env"
+
     def build(self, elaboratable, name="top",
               build_dir="build", do_build=True,
               program_opts=None, do_program=False,
               **kwargs):
-        for tool in self.required_tools:
-            require_tool(tool)
+        if self._toolchain_env_var not in os.environ:
+            for tool in self.required_tools:
+                require_tool(tool)
 
         plan = self.prepare(elaboratable, name, **kwargs)
         if not do_build:
@@ -78,6 +83,8 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         self.toolchain_program(products, name, **(program_opts or {}))
 
     def has_required_tools(self):
+        if self._toolchain_env_var in os.environ:
+            return True
         return all(has_tool(name) for name in self.required_tools)
 
     @abstractmethod
@@ -238,13 +245,13 @@ class TemplatedPlatform(Platform):
         "build_{{name}}.sh": """
             # {{autogenerated}}
             set -e{{verbose("x")}}
-            [ -n "$NMIGEN_{{platform.toolchain}}_env" ] && . "$NMIGEN_{{platform.toolchain}}_env"
+            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
             {{emit_commands("sh")}}
         """,
         "build_{{name}}.bat": """
             @rem {{autogenerated}}
             {{quiet("@echo off")}}
-            if defined NMIGEN_{{platform.toolchain}}_env call %NMIGEN_{{platform.toolchain}}_env%
+            if defined {{platform._toolchain_env_var}} call %{{platform._toolchain_env_var}}%
             {{emit_commands("bat")}}
         """,
     }
diff --git a/nmigen/vendor/lattice_ecp5.py b/nmigen/vendor/lattice_ecp5.py
index db4ac06..c02e370 100644
--- a/nmigen/vendor/lattice_ecp5.py
+++ b/nmigen/vendor/lattice_ecp5.py
@@ -163,9 +163,9 @@ class LatticeECP5Platform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            if [ -n "$NMIGEN_{{platform.toolchain}}_env" ]; then
-                bindir=$(dirname "$NMIGEN_{{platform.toolchain}}_env")
-                . "$NMIGEN_{{platform.toolchain}}_env"
+            if [ -n "${{platform._toolchain_env_var}}" ]; then
+                bindir=$(dirname "${{platform._toolchain_env_var}}")
+                . "${{platform._toolchain_env_var}}"
             fi
             {{emit_commands("sh")}}
         """,
diff --git a/nmigen/vendor/xilinx_spartan_3_6.py b/nmigen/vendor/xilinx_spartan_3_6.py
index c4c18ce..00d6831 100644
--- a/nmigen/vendor/xilinx_spartan_3_6.py
+++ b/nmigen/vendor/xilinx_spartan_3_6.py
@@ -88,7 +88,7 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            [ -n "$NMIGEN_{{platform.toolchain}}_env" ] && . "$NMIGEN_{{platform.toolchain}}_env"
+            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""