build: fix multiple CVEs by upgrading to go1.26.6

- CVE-2026-56860: net/url: quadratic complexity in resolvePath
- CVE-2026-56858: html/template: JavaScript regexp context tracking
- CVE-2026-56862: crypto/tls: limit handshake messages accepted post-handshake
- CVE-2026-56853: net/http: apply ReadHeaderTimeout to unencrypted HTTP/2 check
- CVE-2026-56859: encoding/xml: recursion depth guard during decode
- CVE-2026-33818: encoding/asn1: enforce maximum recursion depth
- CVE-2026-46600: net: panic parsing an invalid SVCB or HTTPS RR in dnsmessage
- CVE-2026-39821: net/http: reject ASCII-only Punycode-encoded labels in idna

This also updates the go1.25 test job to go1.25.13 which contains the
same fixes.

.github/workflows/build.yml

@@ -37,7 +37,7 @@ jobs:
         include:
           - job_name: linux
             os: ubuntu-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             gotags: cmount
             build_flags: '-include "^linux/"'
             check: true
@@ -48,14 +48,14 @@ jobs:
 
           - job_name: linux_386
             os: ubuntu-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             goarch: 386
             gotags: cmount
             quicktest: true
 
           - job_name: mac_amd64
             os: macos-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             gotags: 'cmount'
             build_flags: '-include "^darwin/amd64" -cgo'
             quicktest: true
@@ -64,14 +64,14 @@ jobs:
 
           - job_name: mac_arm64
             os: macos-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             gotags: 'cmount'
             build_flags: '-include "^darwin/arm64" -cgo -macos-arch arm64 -cgo-cflags=-I/usr/local/include -cgo-ldflags=-L/usr/local/lib'
             deploy: true
 
           - job_name: windows
             os: windows-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             gotags: cmount
             cgo: '0'
             build_flags: '-include "^windows/"'
@@ -81,14 +81,14 @@ jobs:
 
           - job_name: other_os
             os: ubuntu-latest
-            go: '~1.26.5'
+            go: '~1.26.6'
             build_flags: '-exclude "^(windows/|darwin/|linux/)"'
             compile_all: true
             deploy: true
 
           - job_name: go1.25
             os: ubuntu-latest
-            go: '~1.25.12'
+            go: '~1.25.13'
             quicktest: true
             racequicktest: true
 
@@ -283,7 +283,7 @@ jobs:
         id: setup-go
         uses: actions/setup-go@v7
         with:
-          go-version: '~1.26.5'
+          go-version: '~1.26.6'
           check-latest: true
           cache: false
           
@@ -398,7 +398,7 @@ jobs:
         id: setup-go
         uses: actions/setup-go@v7
         with:
-          go-version: '~1.26.5'
+          go-version: '~1.26.6'
           # Caching is handled explicitly below to share the module cache
           # with the other jobs - see the build job for the rationale.
           cache: false