feat: support overriding gnu mirror source

Use `https://mirrors.kernel.org/gnu/` as default.

Signed-off-by: Noel Georgi <git@frezbo.dev>

Author: frezbo

.kres.yaml

@@ -98,6 +98,7 @@ spec:
     - TOOLS
     - TOOLS_PREFIX
     - TEST_STAGE
+    - GNU_MIRROR_URL
   makefile:
     extraVariables:
       - name: EXTENSIONS_IMAGE_REF
@@ -112,6 +113,8 @@ spec:
         defaultValue: ghcr.io/siderolabs
       - name: GO_TOOLS_RELEASE
         defaultValue: v0.3.1
+      - name: GNU_MIRROR_URL
+        defaultValue: https://mirrors.kernel.org
   useBldrPkgTagResolver: true
 ---
 kind: common.Build

Makefile

@@ -1,6 +1,6 @@
 # THIS FILE WAS AUTOMATICALLY GENERATED BY KRES, PLEASE DO NOT EDIT.
 #
-# Generated on 2026-07-30T16:21:31Z by kres 34f68df.
+# Generated on 2026-08-05T10:23:44Z by kres 93d28ca.
 
 # common variables
 
@@ -46,6 +46,7 @@ BUILD_ARGS += --build-arg=PKGS_PREFIX="$(PKGS_PREFIX)"
 BUILD_ARGS += --build-arg=TOOLS="$(TOOLS)"
 BUILD_ARGS += --build-arg=TOOLS_PREFIX="$(TOOLS_PREFIX)"
 BUILD_ARGS += --build-arg=TEST_STAGE="$(TEST_STAGE)"
+BUILD_ARGS += --build-arg=GNU_MIRROR_URL="$(GNU_MIRROR_URL)"
 COMMON_ARGS = --file=Pkgfile
 COMMON_ARGS += --provenance=false
 COMMON_ARGS += --sbom=false
@@ -61,6 +62,7 @@ PKGS_PREFIX ?= ghcr.io/siderolabs
 TOOLS ?= v1.14.0-alpha.0-26-g08071b1
 TOOLS_PREFIX ?= ghcr.io/siderolabs
 GO_TOOLS_RELEASE ?= v0.3.1
+GNU_MIRROR_URL ?= https://mirrors.kernel.org
 
 # targets defines all the available targets
 

misc/glibc/pkg.yaml

@@ -17,7 +17,7 @@ steps:
       LD_LINUX_X86_64: ld-linux-x86-64.so.2
       LD_LINUX_AARCH64: ld-linux-aarch64.so.1
     sources:
-      - url: https://ftpmirror.gnu.org/gnu/glibc/glibc-{{ .GLIBC_VERSION }}.tar.xz
+      - url: "{{ .GNU_MIRROR_URL }}/gnu/glibc/glibc-{{ .GLIBC_VERSION }}.tar.xz"
         destination: glibc.tar.xz
         sha256: {{ .GLIBC_SHA256 }}
         sha512: {{ .GLIBC_SHA512 }}

vars.yaml

@@ -0,0 +1 @@
+GNU_MIRROR_URL: "{{ .BUILD_ARG_GNU_MIRROR_URL }}"