cmd/go: fix toolchain escape via PATH in covermode

[zccold]

Fixes #77820

When a user has a local Go installation of 1.25.5 but their go.mod
specifies 1.26.0, running a test command with coverage flags like
go test -covermode=count -coverpkg=./... ./... triggers the
GOTOOLCHAIN auto-switch mechanism to use the newer toolchain.

However, b.CovData uses the hardcoded 'go' command to invoke the
builtin tool covdata. Because GOTOOLCHAIN does not modify the system
PATH, exec.LookPath resolves this hardcoded 'go' to the host's older
Go version (e.g., 1.25.5).

Since covdata is a builtin tool built and run dynamically by the go
command, invoking the older go binary to handle the new toolchain's
files causes a fatal mismatch. The build fails with an error like:
compile: version "go1.26.0" does not match go tool version "go1.25.5..."

This CL fixes the issue by replacing the hardcoded 'go' with the
absolute path to the current toolchain's go executable, ensuring the
correct toolchain context is maintained and preventing PATH escape.

[gopherbot]

This PR (HEAD: 050e88c) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/749360.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/749360.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://go.dev/s/release
for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/749360.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from ZC cold:

Patch Set 1:

(2 comments)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/749360.
After addressing review feedback, remember to publish your drafts!