[lit] Try to fix new test from 28412d1800 under Windows

`llvm/utils/lit/tests/Inputs/shtest-define/value-escaped.txt` broke at
least at <https://lab.llvm.org/buildbot/#/builders/216/builds/10114>.

The problem appears to be a non-portable `echo` command line.

llvm/utils/lit/tests/Inputs/shtest-define/lit.cfg

@@ -1,7 +1,10 @@
 import lit.formats
 config.name = 'shtest-define'
 config.suffixes = ['.txt']
-config.test_format = lit.formats.ShTest()
+# Use lit's internal shell to avoid shell portability issues within RUN lines
+# (e.g., for 'echo' commands in Windows).  Those issues should be orthogonal to
+# the substitution behavior we are trying to test.
+config.test_format = lit.formats.ShTest(execute_external=False)
 config.test_source_root = None
 config.test_exec_root = None