patch 8.2.4754: using cached values after unsetting some environment variables

Problem:    Still using cached values after unsetting some known environment
            variables.
Solution:   Take care of the side effects. (closes #10194)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index da2822c..afa0dcf 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2656,6 +2656,7 @@
 	    case ISN_SOURCE:
 		{
 		    int notused;
+
 		    SOURCING_LNUM = iptr->isn_lnum;
 		    if (may_load_script((int)iptr->isn_arg.number, &notused)
 								       == FAIL)
@@ -3490,7 +3491,7 @@
 		    goto on_error;
 		break;
 	    case ISN_UNLETENV:
-		vim_unsetenv(iptr->isn_arg.unlet.ul_name);
+		vim_unsetenv_ext(iptr->isn_arg.unlet.ul_name);
 		break;
 
 	    case ISN_LOCKUNLOCK: