Switch back to master branch for Ski repo

This branch has an updated memory map that should work with FLATMEM w/o
the following extra change to Linux:
```diff
diff --git a/arch/ia64/mm/contig.c b/arch/ia64/mm/contig.c
index 1e9eaa1..7e26cd17d 100644
--- a/arch/ia64/mm/contig.c
+++ b/arch/ia64/mm/contig.c
@@ -180,10 +180,12 @@ static void __init verify_gap_absence(void)

 	/* Forbid FLATMEM if hole is > than 1G */
 	efi_memmap_walk(find_largest_hole, (u64 *)&max_gap);
+	/*
 	if (max_gap >= SZ_1G)
 		panic("Cannot use FLATMEM with %ldMB hole\n"
 		      "Please switch over to SPARSEMEM\n",
 		      (max_gap >> 20));
+	*/
 }

 /*
```
which is no longer in the Ski/HP-Sim patch set starting with 7.0, which
would explain, why this issue actually started with 7.0.

But why there's no output showing that error:
```
[    0.000000] Kernel panic - not syncing: Cannot use FLATMEM with 3968MB hole
[    0.000000] Please switch over to SPARSEMEM
```
...is still unknown, as `bski` should still create a logfile and `tail`
should show its content.

Author: johnny-mnemonic

.github/workflows/mirror.yml

@@ -94,7 +94,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           repository: johnny-mnemonic/ski
-          ref: update-processor
+          ref: master
           path: ski
 
       - name: prepare_step