Move Linux impeller_unittests to linux_unopt (#50558)

This PR moves linux impeller_unittests to the correct builder, but does
not yet merge it with the rest of the tests running in a
host_debug_unopt build due to:
flutter/flutter#143330

Author: zanderso

ci/builders/linux_host_engine.json

@@ -1,45 +1,5 @@
 {
     "builds": [
-        {
-            "archives": [],
-            "drone_dimensions": [
-                "device_type=none",
-                "os=Linux"
-            ],
-            "gclient_variables": {
-                "download_android_deps": false
-            },
-            "gn": [
-                "--runtime-mode",
-                "debug",
-                "--unoptimized",
-                "--prebuilt-dart-sdk",
-                "--target-dir",
-                "host_debug_impeller_vulkan"
-            ],
-            "name": "host_debug_impeller_vulkan",
-            "ninja": {
-                "config": "host_debug_impeller_vulkan",
-                "targets": [
-                    "flutter",
-                    "flutter/sky/packages"
-                ]
-            },
-            "tests": [
-                {
-                    "language": "python3",
-                    "name": "Host Tests for host_debug_impeller_vulkan",
-                    "script": "flutter/testing/run_tests.py",
-                    "parameters": [
-                        "--variant",
-                        "host_debug_impeller_vulkan",
-                        "--type",
-                        "impeller",
-                        "--engine-capture-core-dump"
-                    ]
-                }
-            ]
-        },
         {
             "archives": [
                 {

ci/builders/linux_unopt.json

@@ -81,6 +81,50 @@
                 }
             ]
         },
+        {
+            "archives": [],
+            "drone_dimensions": [
+                "device_type=none",
+                "os=Linux",
+                "cores=32"
+            ],
+            "gclient_variables": {
+                "download_android_deps": false,
+                "use_rbe": true
+            },
+            "gn": [
+                "--runtime-mode",
+                "debug",
+                "--unoptimized",
+                "--prebuilt-dart-sdk",
+                "--target-dir",
+                "host_debug_unopt_impeller_tests",
+                "--rbe",
+                "--no-goma"
+            ],
+            "name": "host_debug_unopt_impeller_tests",
+            "ninja": {
+                "config": "host_debug_unopt_impeller_tests",
+                "targets": [
+                    "flutter",
+                    "flutter/sky/packages"
+                ]
+            },
+            "tests": [
+                {
+                    "language": "python3",
+                    "name": "Host Tests for host_debug_unopt_impeller_tests",
+                    "script": "flutter/testing/run_tests.py",
+                    "parameters": [
+                        "--variant",
+                        "host_debug_unopt_impeller_tests",
+                        "--type",
+                        "impeller",
+                        "--engine-capture-core-dump"
+                    ]
+                }
+            ]
+        },
         {
             "drone_dimensions": [
                 "device_type=none",