internal/releasetargets: set GOARM=6 for linux-armv6l

From Dmitri's well-researched comment on golang/go#62164:

It was set to GOARM=6 in 1.20 and older because the release target used
the linux-arm-aws builder, and that builder has GOARM=6 in its
environment. For Go 1.21.0, it's built on a linux-amd64 builder via
make.bash -distpack, and GOARM wasn't set in the release target's
environment explicitly, so it ended up defaulting to 7 in make.bash.

Since this was always the effective environment, set it on the target
going back to 1.19. (I didn't get rid of 1.19 because we're still
planning the out-of-cycle release.)

Fixes golang/go#62164

Change-Id: I539d9dfaf9036e4ac9a6a11fc1e73440d8437a80
Reviewed-on: https://go-review.googlesource.com/c/build/+/521555
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/releasetargets/releases.txt

@@ -28,6 +28,7 @@ linux-arm64     linux      arm64      linux-arm64
 	Running on GCP
 
 linux-armv6l    linux      arm        linux-arm-aws
+	Extra env: ["GOARM=6"]
 	Running on AWS
 
 linux-ppc64le   linux      ppc64le    linux-ppc64le-buildlet
@@ -78,6 +79,7 @@ linux-arm64     linux      arm64      linux-arm64
 	Running on GCP
 
 linux-armv6l    linux      arm        linux-arm-aws
+	Extra env: ["GOARM=6"]
 	Running on AWS
 
 linux-ppc64le   linux      ppc64le    linux-ppc64le-buildlet
@@ -140,6 +142,7 @@ linux-arm64     linux      arm64      linux-arm64
 	Running on GCP
 
 linux-armv6l    linux      arm        linux-arm-aws
+	Extra env: ["GOARM=6"]
 	Running on AWS
 
 linux-loong64   linux      loong64    (cross-compiled, no tests)

internal/releasetargets/releasetargets.go

@@ -87,8 +87,9 @@ var allReleases = map[int]ReleaseTargets{
 			LongTestBuilder: "linux-386-longtest",
 		},
 		"linux-armv6l": &Target{
-			GOARCH:  "arm",
-			Builder: "linux-arm-aws",
+			GOARCH:   "arm",
+			Builder:  "linux-arm-aws",
+			ExtraEnv: []string{"GOARM=6"},
 		},
 		"linux-arm64": &Target{
 			Builder: "linux-arm64",