CI: Update release jobs to use go1.25.8 and go1.26.1

[beautifulentropy]

This was missed in #8664. Also, update the pinned signing key used to verify Go tarball signatures and add instructions for how to do this in the future.

---

Here is the output from my new instructions:

$ export GNUPGHOME="$(mktemp -d)"

$ pbpaste > old

$ gpg --show-keys --with-fingerprint old
pub   dsa1024 2007-03-08 [SC]
      4CCA 1EAF 950C EE4A B839  76DC A040 830F 7FAC 5991
uid                      Google, Inc. Linux Package Signing Key <linux-packages-keymaster@google.com>
sub   elg2048 2007-03-08 [E]

pub   rsa4096 2016-04-12 [SC]
      EB4C 1BFD 4F04 2F6D DDCC  EC91 7721 F63B D38B 4796
uid                      Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>
sub   rsa4096 2016-04-12 [S] [expired: 2019-04-12]
sub   rsa4096 2017-01-24 [S] [expired: 2020-01-24]
sub   rsa4096 2019-07-22 [S] [expired: 2022-07-21]
sub   rsa4096 2021-10-26 [S] [expired: 2024-10-25]
sub   rsa4096 2023-02-15 [S] [expired: 2026-02-14]
sub   rsa4096 2024-01-30 [S] [expires: 2027-01-29]

$ curl -fsSL https://dl.google.com/linux/linux_signing_key.pub -o new

$ gpg --show-keys --with-fingerprint --with-subkey-fingerprint new
pub   rsa4096 2016-04-12 [SC]
      EB4C 1BFD 4F04 2F6D DDCC  EC91 7721 F63B D38B 4796
uid                      Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>
sub   rsa4096 2016-04-12 [S] [expired: 2019-04-12]
      3B06 8FB4 789A BE4A EFA3  BB49 1397 BC53 640D B551
sub   rsa4096 2017-01-24 [S] [expired: 2020-01-24]
      3E50 F6D3 EC27 8FDE B655  C8CA 6494 C6D6 997C 215E
sub   rsa4096 2019-07-22 [S] [expired: 2022-07-21]
      2F52 8D36 D67B 69ED F998  D857 78BD 6547 3CB3 BD13
sub   rsa4096 2021-10-26 [S] [expired: 2024-10-25]
      8461 EFA0 E74A BAE0 10DE  6699 4EB2 7DB2 A3B8 8B8B
sub   rsa4096 2023-02-15 [S] [expired: 2026-02-14]
      A5F4 83CD 733A 4EBA EA37  8B2A E889 79FB 9B30 ACF2
sub   rsa4096 2024-01-30 [S] [expires: 2027-01-29]
      0F06 FF86 BEEA F4E7 1866  EE52 32EE 5355 A6BC 6E42
sub   rsa4096 2025-01-07 [S] [expires: 2028-01-07]
      0E22 5917 4146 70F4 442C  250D FD53 3C07 C264 648F

$ gpg --import old
gpg: /var/folders/ny/j6nmjsvd52n4d28_zn2y9fxw0000gn/T/tmp.bx8BwTtBRr/trustdb.gpg: trustdb created
gpg: key A040830F7FAC5991: public key "Google, Inc. Linux Package Signing Key <linux-packages-keymaster@google.com>" imported
gpg: key 7721F63BD38B4796: public key "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" imported
gpg: Total number processed: 2
gpg:               imported: 2

$ gpg --import new
gpg: key 7721F63BD38B4796: "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" 1 new signature
gpg: key 7721F63BD38B4796: "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" 1 new subkey
gpg: Total number processed: 1
gpg:            new subkeys: 1
gpg:         new signatures: 1