Cirrus: Fix gate image & false-positive exits

A number of scripts relating to tooling used and the gate container
image were not exiting upon errors as intended.  Coupled with
external service unavailability (i.e. downloading golangci-lint)
was observed to cause difficult to debug failures.

This change corrects the scripts inside/out of the gate container as
well as fixes many golang related path consistency problems vs other CI
jobs.  After this change, all jobs use consistent path names reducing
the number of special-case overrides needed.

Lastly, I also made a documentation-pass, updating/correcting as needed,
including documenting a likely local validation-failure mode, related to
`$EPOCH_TEST_COMMIT`.  This is dependent on the developers git
environment, so documentation is the only possible "fix".

Signed-off-by: Chris Evich <cevich@redhat.com>

hack/install_golangci.sh

@@ -1,17 +1,17 @@
 #!/bin/bash
 
-if [ -z "$VERSION" ]; then
-	echo \$VERSION is empty
-	exit 1
-fi
+set -e
 
-if [ -z "$GOBIN" ]; then
-	echo \$GOBIN is empty
-	exit 1
-fi
+die() { echo "${1:-No error message given} (from $(basename $0))"; exit 1; }
 
-$GOBIN/golangci-lint --version | grep $VERSION
-if [ $?  -ne 0 ]; then
-	set -e
-	curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOBIN v$VERSION
+[ -n "$VERSION" ] || die "\$VERSION is empty or undefined"
+[ -n "$GOBIN" ] || die "\$GOBIN is empty or undefined"
+
+BIN="$GOBIN/golangci-lint"
+if [ ! -x "$BIN" ]; then
+    echo "Installing golangci-lint v$VERSION into $GOBIN"
+    curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOBIN v$VERSION
+else
+    # Prints it's own file name as part of --verison output
+    echo "Using existing $(dirname $BIN)/$($BIN --version)"
 fi