docs/anv: fix envvar documentation

It was accidentally trying to link to an envvar definition instead of creating said definition.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28499>

docs/drivers/anv.rst

@@ -7,26 +7,37 @@ Debugging
 Here are a few environment variable debug environment variables
 specific to ANV:
 
-:envvar:`ANV_ENABLE_PIPELINE_CACHE`
+.. envvar:: ANV_ENABLE_PIPELINE_CACHE
+
    If defined to ``0`` or ``false``, this will disable pipeline
    caching, forcing ANV to reparse and recompile any VkShaderModule
    (SPIRV) it is given.
-:envvar:`ANV_DISABLE_SECONDARY_CMD_BUFFER_CALLS`
+
+.. envvar:: ANV_DISABLE_SECONDARY_CMD_BUFFER_CALLS
+
    If defined to ``1`` or ``true``, this will prevent usage of self
    modifying command buffers to implement ``vkCmdExecuteCommands``. As
    a result of this, it will also disable :ext:`VK_KHR_performance_query`.
-:envvar:`ANV_ALWAYS_BINDLESS`
+
+.. envvar:: ANV_ALWAYS_BINDLESS
+
    If defined to ``1`` or ``true``, this forces all descriptor sets to
    use the internal `Bindless model`_.
-:envvar:`ANV_QUEUE_THREAD_DISABLE`
+
+.. envvar:: ANV_QUEUE_THREAD_DISABLE
+
    If defined to ``1`` or ``true``, this disables support for timeline
    semaphores.
-:envvar:`ANV_USERSPACE_RELOCS`
+
+.. envvar:: ANV_USERSPACE_RELOCS
+
    If defined to ``1`` or ``true``, this forces ANV to always do
    kernel relocations in command buffers. This should only have an
    effect on hardware that doesn't support soft-pinning (Ivybridge,
    Haswell, Cherryview).
-:envvar:`ANV_PRIMITIVE_REPLICATION_MAX_VIEWS`
+
+.. envvar:: ANV_PRIMITIVE_REPLICATION_MAX_VIEWS
+
    Specifies up to how many view shaders can be lowered to handle
    :ext:`VK_KHR_multiview`. Beyond this number, multiview is implemented
    using instanced rendering. If unspecified, the value default to