[workflows] Fix lldb-tests and libclc-tests (llvm#80751)

This was broken by d25022b, which
caused the workflow to pass an empty string to ninja as the target. The
'all' target is probably not the right target for these tests, but this
is what the behavior was before
d25022b.

(cherry picked from commit 792d928)

Author: tstellar

.github/workflows/libclc-tests.yml

@@ -36,5 +36,4 @@ jobs:
     name: Test libclc
     uses: ./.github/workflows/llvm-project-tests.yml
     with:
-      build_target: ''
       projects: clang;libclc

.github/workflows/lldb-tests.yml

@@ -36,5 +36,4 @@ jobs:
     name: Build lldb
     uses: ./.github/workflows/llvm-project-tests.yml
     with:
-      build_target: ''
       projects: clang;lldb

.github/workflows/llvm-project-tests.yml

@@ -22,8 +22,9 @@ on:
   workflow_call:
     inputs:
       build_target:
-        required: true
+        required: false
         type: string
+        default: "all"
 
       projects:
         required: true