TUN-10507: Bump go and go-boring to 1.26.2

## What

Bumps go-boring from 1.26.0-1 to 1.26.2-1 and CI builder image from \`3501-fc698419a625\` to \`3595-779e088c0ec4\`.

go1.26.2 (released 2026-04-07) includes security fixes to the \`go\` command, the compiler, and the \`archive/tar\`, \`crypto/tls\`, \`crypto/x509\`, \`html/template\`, and \`os\` packages, as well as bug fixes to the \`net\`, \`net/http\`, and \`net/url\` packages.

### Security fixes (relevant)
- **crypto/tls**: multiple CVEs — cloudflared uses TLS extensively for tunnel connections
- **crypto/x509**: CVE-2026-32280 (excessive chain-building in \`Verify\`), CVE-2026-32281 (quadratic work in policy validation)

### Net bug fixes (not applicable)
- **net/url #78111**: \`url.Parse\` regression for MongoDB-style multi-host URLs — not used in cloudflared
- **net/http #78019**: race condition on Windows when using \`os.File\` as HTTP request body — cloudflared does not pass \`os.File\` as a request body
- **net #77885**: \`ReadMsgUDP\`/\`WriteMsgUDP\` WSAEFAULT on Windows with empty non-nil oob — quic-go uses \`basicConn\` on Windows (\`ReadFrom\`, not \`ReadMsgUDP\`)

## Jira

[TUN-10507](https://jira.cfdata.org/browse/TUN-10507)

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 variables:
-  GO_VERSION: "1.26.0"
+  GO_VERSION: "1.26.2"
   MAC_GO_VERSION: "go@$GO_VERSION"
   WIN_GO_VERSION: "go$GO_VERSION"
   GIT_DEPTH: "0"