Migrate GPU test jobs to pkgci. (#18007)

Progress on https://github.com/iree-org/iree/issues/16203. Depends on
https://github.com/iree-org/iree/pull/18000.

These jobs used to use the 3.2GB install directory produced by `cmake
--build full-build-dir --target install` in the `build_all` job. Now
they use the 73MB Python packages produced by `python -m pip wheel
runtime/` and `python -m pip wheel compiler/` in the `build_packages`
job. Python packages are what we expect users to consume, so test jobs
should use them too.
* Note that the Python packages will be larger once we enable asserts
and/or debug symbols in them. These tests may also fail with less useful
error messages and callstacks as a result of this change until that is
fixed.

I tried to keep changes to the workflow jobs minimal for now. Once the
migrations are further along we can cut out some of the remaining layers
of scripts / Dockerfiles. As before, these jobs are all opt-in on
presubmit (always running on LLVM integrate PRs or PRs affecting
NVGPU/AMDGPU code). Diffs between previous jobs and new jobs to confirm
how similar they are:

Job name | Logs before | Logs after | Notes
-- | -- | -- | --
`test_nvidia_t4` | [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102664951/job/27939423899)
| [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102841435/job/27939725841?pr=18007)
| 433 tests -> 430 tests<br>skipping
`samples/custom_dispatch/vulkan/shaders`<br>`IREE
custom_dispatch/vulkan/shaders ignored -- glslc not found`<br>(no longer
running under Docker)
`test_amd_mi250` | [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102664951/job/27939423747)
| [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102841435/job/27939725347?pr=18007)
| 138 tests before/after
`test_amd_mi300` | [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102664951/job/27939424223)
| [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102841435/job/27939725525?pr=18007)
| 141 tests before/after
`test_amd_w7900` | [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102664951/job/27939424084)
| [workflow
logs](https://github.com/iree-org/iree/actions/runs/10102841435/job/27939725679?pr=18007)
| 289 tests before/after

Each job is now included from its own standalone workflow file, allowing
for testing of individual workflows using `workflow_dispatch` triggers.
I have some other ideas for further decoupling these optional jobs from
the core workflow(s).

ci-extra: test_amd_mi250, test_amd_mi300, test_amd_w7900, test_nvidia_t4
diff --git a/build_tools/cmake/iree_lit_test.cmake b/build_tools/cmake/iree_lit_test.cmake
index f93d84d..46f8622 100644
--- a/build_tools/cmake/iree_lit_test.cmake
+++ b/build_tools/cmake/iree_lit_test.cmake
@@ -18,8 +18,8 @@
 # TOOLS: Tools that should be included on the PATH
 # DATA: Additional data dependencies invoked by the test (e.g. binaries
 #   called in the RUN line)
-# LABELS: Additional labels to apply to the test. The package path is added
-#     automatically.
+# LABELS: Additional labels to apply to the test. Package path and
+#     "test-type=lit-test" labels are added automatically.
 function(iree_lit_test)
   if(NOT IREE_BUILD_TESTS)
     return()
@@ -75,6 +75,7 @@
   )
 
   list(APPEND _RULE_LABELS "${_PACKAGE_PATH}")
+  list(APPEND _RULE_LABELS "test-type=lit-test")
   set_property(TEST ${_NAME_PATH} PROPERTY LABELS "${_RULE_LABELS}")
   set_property(TEST ${_NAME_PATH} PROPERTY REQUIRED_FILES "${_TEST_FILE_PATH}")
   set_property(TEST ${_NAME_PATH} PROPERTY ENVIRONMENT