Use fixed tag for forked golangci-lint in unstable

Instead of silently using the very latest commit from the
`feat/go1.23` branch on the fork use a fixed tag to make
it easier to track when unstable golangci-lint builds are
updated.

refs GH-1591

unstable/combined/Dockerfile

@@ -19,7 +19,7 @@ ENV GOTOOLCHAIN="local"
 # versions, otherwise the latest upstream build of the tool is installed in
 # this image.
 # ENV GOLANGCI_LINT_VERSION="v1.59.1"
-ENV GOLANGCI_LINT_VERSION="feat/go1.23"
+ENV GOLANGCI_LINT_VERSION="FORK-go1.23-support-2024-07-18"
 
 # A current master branch build is used for pre-release Go versions, otherwise
 # the latest upstream build of the tool is installed in this image.
@@ -127,7 +127,7 @@ ENV GOTOOLCHAIN="local"
 # versions, otherwise the latest upstream build of the tool is installed in
 # this image.
 # ENV GOLANGCI_LINT_VERSION="v1.59.1"
-ENV GOLANGCI_LINT_VERSION="feat/go1.23"
+ENV GOLANGCI_LINT_VERSION="FORK-go1.23-support-2024-07-18"
 
 # ENV STATICCHECK_VERSION="v0.4.7"
 # ENV STATICCHECK_VERSION="dec278f2f0d94b07c04db075d807e9f499f5d7b5"